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91-522: While many calendar reforms aim to make the calendar more accurate, the Hanke–Henry Permanent Calendar focuses on making the calendar perennial , so that every date falls on the same day of the week, year after year. The familiar drift of weekdays concerning dates results from the fact that the number of days in a physical year (one full orbit of Earth around the Sun, approximately 365.24 days)

182-549: A comma or a dot on the baseline , is used as a separator between the time element and its fraction. (Following ISO 80000-1 according to ISO 8601:1-2019, it does not stipulate a preference except within International Standards, but with a preference for a comma according to ISO 8601:2004. ) For example, to denote "14 hours, 30 and one half minutes", do not include a seconds figure; represent it as "14:30,5", "T1430,5", "14:30.5", or "T1430.5". There

273-409: A comma or a full stop , as in "P0,5Y" or "P0.5Y". The standard does not prohibit date and time values in a duration representation from exceeding their "carry over points" except as noted below. Thus, "PT36H" could be used as well as "P1DT12H" for representing the same duration. But keep in mind that "PT36H" is not the same as "P1DT12H" when switching from or to Daylight saving time . Alternatively,

364-740: A Thursday on the corresponding Gregorian calendar, and falls between the end of December and the beginning of January. Thus, each year always begins between December 29 and January 4 in the Gregorian calendar. This is effectively the same rule as in ISO week dates . Under the Hanke–Henry Permanent Calendar January, February, April, May, July, August, October, and November have thirty days, while March, June, September, and December have thirty-one so that each quarter contains two 30-day months followed by one month of 31 days (30:30:31). While

455-600: A better fit to the sidereal year. There have been reforms of the solar version of the Hindu calendar which changed the distribution of the days in each month to better match the length of time that the Sun spends in each sidereal zodiacal sign. The same applies to the Buddhist calendar . The first millennium reform of the Hebrew calendar changed it from an observational calendar into a calculated calendar. The Islamic calendar

546-421: A calendar. Most cultures adopt a primary dating system, but different cultures have always needed to align multiple calendars with each other, either because they coexisted in the same space (e.g. secular and religious groups with different demands) or had established trading relations. Once specified, a calendar design cannot be altered without becoming a new design. If a proposed design is sufficiently close to

637-491: A complete date expression, the letter "T" as a delimiter, and a valid time expression. For example, "2007-04-05T14:30" . In ISO 8601:2004 it was permitted to omit the "T" character by mutual agreement as in "200704051430" , but this provision was removed in ISO ;8601-1:2019. Separating date and time parts with other characters such as space is not allowed in ISO 8601, but allowed in its profile RFC 3339. If

728-411: A device to track solar day-to-day progression, such as that established at places like Stonehenge . After centuries of empirical observations, the theoretical aspects of calendar construction could become more refined, enabling predictions that identified the need for reform. There have been 50 to 100 reforms of the traditional Chinese calendar over 2500 years, most of which were intended to better fit

819-407: A format for duration based on combined date and time representations may be used by agreement between the communicating parties either in the basic format PYYYYMMDDThhmmss or in the extended format P[YYYY]-[MM]-[DD]T[hh]:[mm]:[ss] . For example, the first duration shown above would be "P0003-06-04T12:30:05" . However, individual date and time values cannot exceed their moduli (e.g. a value of 13 for

910-508: A four-digit year, 0000 through 9999. [MM] indicates a two-digit month of the year, 01 through 12. [DD] indicates a two-digit day of that month, 01 through 31. For example, "5 April 1981" may be represented as either "1981-04-05" in the extended format or "19810405" in the basic format . The standard also allows for calendar dates to be written with reduced precision. For example, one may write "1981-04" to mean "1981 April". One may simply write "1981" to refer to that year, "198" to refer to

1001-431: A leap month called Meton every 3 or 2 years with 30 or 31 days. 60 years together are called a cycle. It uses a leap cycle which has equal number of days, weeks, months, years and cycles. 2498258 days, 356894 weeks, 84599 months, 6840 years and 114 cycles nearly all equal each other. It is called an era, although time isn't divided into it in this calendar. Some propose to improve leap rules of existing calendars, such as

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1092-482: A leap week of seven days to the calendar every five or six years to keep the calendar roughly in step with the tropical year. They have years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week. Proposals mainly differ in their selection of a leap rule, placing of the leap item (usually middle or end of the year), in the start day of the week and year, in the number (12 or 13) and size of months and in connected naming; some are compatible to

1183-422: A leap week, but years whose numbers are divisible by 40 did not unless they are also divisible by 400. Henry had advocated transition to the calendar on 1 January 2006 as that is a year in which his calendar and the Gregorian calendar begin the year on the same day. After that date passed, he recommended dropping off 31 December 2006 to start in 2007, or dropping 30 and 31 December 2007 to start 2008. In late 2011

1274-492: A positive UTC offset describes a time zone east of the prime meridian where the civil time is ahead of UTC . So the zone designation for Cairo will be "+02:00","+0200", or "+02". A time zone where the civil time coincides with UTC is always designated as positive, though the offset is zero (see related specifications below). So the zone designation for London (on standard time ) would be " +00:00 ", " +0000 ", or " +00 ". See List of UTC offsets for other UTC offsets . It

1365-521: A sabbath every seven days. Independently the World Council of Churches still tries to find a common rule for the date of Easter, which might be eased by a new common calendar. Reformers cite several problems with the Gregorian calendar: It is hard or even impossible to solve all these issues in just one calendar. Most plans evolve around the solar year of a little more than 365 days. This number does not divide well by seven or twelve, which are

1456-454: A specified numerical meaning within the standard (thus excluding names of years in the Chinese calendar ), or that do not use computer characters (excludes images or sounds). In representations that adhere to the ISO 8601 interchange standard , dates and times are arranged such that the greatest temporal term (typically a year) is placed at the left and each successively lesser term

1547-423: A time zone designator is required, it follows the combined date and time. For example, "2007-04-05T14:30Z" or "2007-04-05T12:30−02:00" . Either basic or extended formats may be used, but both date and time must use the same format. The date expression may be calendar, week, or ordinal, and must use a complete representation. The time may be represented using a specified reduced precision format. Durations define

1638-531: A whole number of days nor a whole number of lunar months; neither does the lunar month have a whole number of days: in each case there are fractions "left over". (In some physical circumstances the rotations and revolutions of a planet and its satellite(s) can be phase-locked – for example the same side of the moon always faces us – but this has not operated to lock together the lengths of the Earth's year, day and month.) Such remainders could accumulate from one period to

1729-399: Is a variant of this concept. Each year of this calendar can be up to 371 days long. Some calendars have quarters of regularly patterned uneven months e.g., a 35-day (five-week) month and a pair of 28-day (four-week) months, with a leap week appended to the final month when needed. The Common Civil Calendar and Time calendar has months of 30 and 31 days, but inserts a leap week in the middle of

1820-507: Is an ordinal format for the multiples of a day elapsed since the start of year. It is represented as "YYYY-DDD" (or YYYYDDD), where [YYYY] indicates a year and [DDD] is the "day of year", from 001 through 365 (366 in leap years ). For example, "1981-04-05" is the same as "1981-095" . This simple form is preferable for occasions when the arbitrary nature of week and month definitions are more of an impediment than an aid, for instance, when comparing dates from different calendars. This format

1911-476: Is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design. The prime objective of a calendar is to unambiguously identify any day in past, present and future by a specific date in order to record or organize social, religious, commercial or administrative events. Recurring periods that contain multiple days, such as weeks , months , and years , are secondary, convenient features of

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2002-488: Is appended directly to the time instead of "Z" suffix above; other nautical time zone letters are not used. The offset is applied to UTC to get the civil time in the designated time zone in the format '±[hh]:[mm]', '±[hh][mm]', or '±[hh]'. A negative UTC offset describes a time zone west of the prime meridian where the civil time is behind UTC. So the zone designation for New York (on standard time ) would be "−05:00","−0500", or "−05". Conversely,

2093-433: Is given with a time representation, the time is assumed to be in local time. While it may be safe to assume local time when communicating in the same time zone, it is ambiguous when used in communicating across different time zones. Even within a single geographic time zone, some local times will be ambiguous if the region observes daylight saving time . It is usually preferable to indicate a time zone (zone designator) using

2184-633: Is maintained by the International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019, and an amendment in 2022. The standard provides a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times. ISO 8601 applies to these representations and formats: dates , in

2275-528: Is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3 for a DATETIME, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]". Time zones in ISO 8601 are represented as local time (with the location unspecified), as UTC , or as an offset from UTC. If no UTC relation information

2366-399: Is not a multiple of seven. By reducing common years to 364 days (52 weeks), and adding an extra week every five or six years, the Hanke–Henry Permanent Calendar eliminates weekday drift and synchronizes the calendar year with the seasonal change as the Earth circles the Sun. The leap week known as "Xtra", occurs every year that either begins ( dominical letters D , DC ) or ends (D, ED ) in

2457-432: Is not a valid representation for a duration of 0 seconds. "PT0S" or "P0D", however, are both valid and represent the same duration. To resolve ambiguity, "P1M" is a one-month duration and "PT1M" is a one-minute duration (note the time designator, T, that precedes the time value). The smallest value used may also have a decimal fraction, as in "P0.5Y" to indicate half a year. This decimal fraction may be specified with either

2548-421: Is not permitted to state a zero value time offset with a negative sign, as "−00:00", "−0000", or "−00". The section dictating sign usage states that a plus sign must be used for a positive or zero value, and a minus sign for a negative value. A plus-minus-sign ( ± ) may also be used if it is available. Contrary to this rule, RFC 3339, which is otherwise a profile of ISO 8601, permits

2639-415: Is placed to the right of the previous term. Representations must be written in a combination of Arabic numerals and the specific computer characters (such as "‐", ":", "T", "W", "Z") that are assigned specific meanings within the standard; that is, such commonplace descriptors of dates (or parts of dates) as "January", "Thursday", or "New Year's Day" are not allowed in interchange representations within

2730-408: Is the weekday number , from 1 through 7, beginning with Monday and ending with Sunday. There are several mutually equivalent and compatible descriptions of week 01: As a consequence, if 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year (there is no week 00). 28 December is always in

2821-484: Is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar has always been outweighed by its inability to track the seasons, and a solar calendar is used in conjunction to remedy this defect. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or

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2912-442: Is used for each season of every year. A decimal calendar is a calendar which includes units of time based on the decimal system . The French Republican Calendar was introduced (along with decimal time ) in 1793. It consisted of twelve months, each divided into three décades of ten days, with five or six intercalary days called sansculottides . The calendar was abolished by Napoleon on January 1, 1806. The lengths of

3003-577: Is used to designate the Zulu time zone . However the ACP 121 standard that defines the list of military time zones makes no mention of UTC and derives the "Zulu time" from the Greenwich Mean Time which was formerly used as the international civil time standard. GMT is no longer precisely defined by the scientific community and can refer to either UTC or UT1 depending on context. The UTC offset

3094-449: Is used with simple hardware systems that have a need for a date system, but where including full calendar calculation software may be a significant nuisance. This system is sometimes referred to as "Julian Date", but this can cause confusion with the astronomical Julian day , a sequential count of the number of days since day 0 beginning 1 January 4713 BC Greenwich noon, Julian proleptic calendar (or noon on ISO date −4713-11-24 which uses

3185-472: The Gregorian calendar (including the proleptic Gregorian calendar); times , based on the 24-hour timekeeping system , with optional UTC offset ; time intervals ; and combinations thereof. The standard does not assign specific meaning to any element of the dates/times represented: the meaning of any element depends on the context of its use. Dates and times represented cannot use words that do not have

3276-409: The Hebrew calendar . The Rectified Hebrew calendar uses a more accurate leap cycle of 4366 months per 353-year cycle, with 130 leap years per cycle, and a progressively shorter molad interval, intended to replace the 19-year leap cycle and the constant molad interval of the traditional fixed arithmetic Hebrew calendar, respectively. Calendar proposals that introduce a thirteenth month or change

3367-480: The Julian calendar would be contrary to the standard (because at the switchover date, the dates would not be consecutive). ISO 8601 prescribes, as a minimum, a four-digit year [YYYY] to avoid the year 2000 problem . It therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others AD , similar to astronomical year numbering . However, years before 1583 (the first full year following

3458-519: The Pax Calendar , which avoids off-calendar days by adding a 7-day leap week to the 364-day common year for 71 out of 400 years. Lunisolar calendars usually have 12 or 13 months of 29 or 30 days. The Hermetic Lunar Week Calendar is a lunisolar calendar proposal which has 12 or 13 lunar months of 29 or 30 days a year, and begins each year near the vernal equinox . The Meyer–Palmen Solilunar Calendar has 12 lunar months with 29 or 30 days plus

3549-520: The decade from 1980 to 1989 inclusive, or "19" to refer to the century from 1900 to 1999 inclusive. Although the standard allows both the "YYYY-MM-DD" and YYYYMMDD formats for complete calendar date representations, if the day [DD] is omitted then only the YYYY-MM format is allowed. By disallowing dates of the form YYYYMM, the standard avoids confusion with the truncated representation YYMMDD (still often used). The 2000 version also allowed writing

3640-768: The week date of ISO 8601. The World Calendar , favored by the UN in the 1950s, and the International Fixed Calendar , quite popular among economists between the World Wars, are proposals that start each year on a Sunday. The 364 days within the week system form 52 weeks of 7 days. The World Calendar has every quarter beginning on the same day of the week. In the World Calendar, the 365th and 366th day are considered holidays and named Worlds Day and Leap Year Day. These "off-calendar" days stand outside

3731-407: The 30th and last day of that month, which makes sense for some feasts like Halloween at least, which should be on the day before All Hallows on the 1st day of November. A third solution, which has been adopted with calendar reforms elsewhere, would be to apply the calendar proleptically and find the corresponding date in the original year, though this would probably have to be done for all dates: e.g.

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3822-476: The 4th of July in 1776 ( Independence Day ) was a Thursday as it is in HHPC, but the 14th of July in 1789 ( Bastille Day ) was on a Tuesday, not a Sunday, and would hence need to be moved to the 16th of July. As part of the calendar proposal, time zones would be eliminated and replaced with Coordinated Universal Time (UTC). Henry argues that his proposal will succeed where some others have failed because it keeps

3913-466: The Gregorian calendar year that is ending; if three, they are Monday, Tuesday and Wednesday. Similarly, the last ISO week of a year may have up to three days that are actually in the Gregorian calendar year that is starting; if three, they are Friday, Saturday, and Sunday. The Thursday of each ISO week is always in the Gregorian calendar year denoted by the ISO week-numbering year. Examples: An ordinal date

4004-506: The Gregorian calendar: The following count one or more days outside the standard seven-day week: The following are leap week calendars: There have also been proposals to revise the way years are numbered: Reform of the Islamic calendar: ISO 8601 2024-11-28T04:30:13+00:00 UTC+00:00 [ refresh ] ISO 8601 is an international standard covering the worldwide exchange and communication of date and time -related data. It

4095-465: The Gregorian leap year rule and has the same cycle length. Years whose numbers are divisible by 5 have a leap week, but years whose numbers are divisible by 40 do not have a leap week unless also divisible by 400. The main drawback of this rule is that the new year varies 17 days relative to the Gregorian new year (e.g. year 1965 begins 11 days earlier than Gregorian 1965 and year 2036 begins 6 days later than Gregorian 2036), whereas Henry's rule ensures that

4186-414: The Gregorian proleptic calendar with a year 0000). ISO 8601 uses the 24-hour clock system. As of ISO 8601-1:2019, the basic format is T[hh][mm][ss] and the extended format is T[hh]:[mm]:[ss]. Earlier versions omitted the T (representing time) in both formats. So a time might appear as either "T134730" in the basic format or "T13:47:30" in the extended format . ISO 8601-1:2019 allows

4277-440: The Hanke–Henry Permanent Calendar changes the length of the months, the week and days remain the same. Hanke and Henry do not offer a serious discussion of anniversaries, especially the ones commemorated on 31 January , 31 May , 31 July , 31 August , and 31 October (as these days are eliminated). Their website FAQ simply recommends to either celebrate one's birthday on a random day of one's choosing, or more systematically use

4368-614: The Julian calendar, although the Russian church year still uses the Julian calendar. Those nations that adopted this calendar on or after 1700, had to drop more than ten days: Great Britain, for instance, dropped eleven. In 1923, Milutin Milanković proposed to a synod of some Eastern Orthodox Churches at Constantinople that only centennial years that leave a remainder of 200 or 600 upon division by 900 would be leap years, decreasing

4459-552: The Julian-Gregorian system of months often also propose new names for these months. New names have also been proposed for days out of the week cycle (e.g., 365th and leap) and weeks out of the month cycle. Proposals to change the traditional month and weekday names are less frequent. The Gregorian calendar obtains its names mostly from gods of historical religions (e.g., Thursday from Nordic Thor or March from Roman Mars ) or leaders of vanished empires (July and August from

4550-482: The T to be omitted in the extended format, as in "13:47:30", but only allows the T to be omitted in the basic format when there is no risk of confusion with date expressions. Either the seconds, or the minutes and seconds, may be omitted from the basic or extended time formats for greater brevity but decreased precision; the resulting reduced precision time formats are: As of ISO 8601-1:2019/Amd 1:2022, "00:00:00" may be used to refer to midnight corresponding to

4641-489: The amount of intervening time in a time interval and are represented by the format P[n]Y[n]M[n]DT[n]H[n]M[n]S or P[n]W as shown on the aside. In these representations, the [n] is replaced by the value for each of the date and time elements that follow the [n]. Leading zeros are not required, but the maximum number of digits for each element should be agreed to by the communicating parties. The capital letters P , Y , M , W , D , T , H , M , and S are designators for each of

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4732-424: The average year length to 365.24 2 days: these remainders were chosen to delay as much as possible the first year (after the year of 1923) that this calendar would disagree with the Gregorian calendar, which occurs in until 2800. It was adopted by some Eastern Orthodox Churches, under the names Revised Julian calendar or New calendar, but was rejected by others. The Gregorian calendar is currently used by most of

4823-457: The calendar months to astronomical lunations and to more accurately add the extra month so that the regular months maintain their proper seasonal positions, even though each seasonal marker can occur anywhere within its month. There have been at least four similar reforms of the lunisolar version of the Hindu calendar , all intended to make the month a better match to the lunation and to make the year

4914-520: The calendar was revised by Johns Hopkins economist Steve Hanke by moving the leap week from the middle to the end of the year and renaming it "Extra", producing the Hanke–Henry Permanent Calendar. The target date for universal adoption was 1 January 2017 then, but was postponed to 2018, when the calendar design was changed in early 2016 to adopt Monday as the start of the week, quarter and year, to better comply with existing international standard ISO 8601 . In 2016, web developer Black Tent Digital released

5005-426: The calendar with the astronomical year (either solar or sidereal ) and/or the synodic month in lunar or lunisolar calendars . Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar , when it was altered to the Gregorian calendar . The fundamental problems of the calendar are that the astronomical year has neither

5096-538: The character set has a minus sign, such as U+2212 − MINUS SIGN in Unicode , then that character should be used. The HTML character entity invocation for − is &minus; . ISO 8601-2:2019 allows for general durations for time offsets. For example, more precision can be added to the time offset with the format '<time>±[hh]:[mm]:[ss].[sss]' or '<time>±[n]H[n]M[n]S' as below. A single point in time can be represented by concatenating

5187-488: The date and time elements and are not replaced. For example, "P3Y6M4DT12H30M5S" represents a duration of "three years, six months, four days, twelve hours, thirty minutes, and five seconds". Date and time elements including their designator may be omitted if their value is zero, and lower-order elements may also be omitted for reduced precision. For example, "P23DT23H" and "P4Y" are both acceptable duration representations. However, at least one element must be present, thus "P"

5278-607: The first Caesars), or ordinals that got out of synchronization (September through December, originally seventh through tenth, now ninth through twelfth). Comte's Positivist calendar, for example, proposed naming the 13 months in his calendar after figures from religion, literature, philosophy and science. Similarly, the Hermetic Lunar Week Calendar uses 12 or 13 lunar months named after 13 contributors to research on psychoactive plants and chemicals. There have been many specific calendar proposals to replace

5369-592: The fourth revision of the standard ISO 8601-1:2019 represents slightly updated contents of the previous ISO 8601:2004 standard, whereas the new ISO 8601-2:2019 defines various extensions such as uncertainties or parts of the Extended Date/Time Format (EDTF). An amendment was published in October 2022 featuring minor technical clarifications and attempts to remove ambiguities in definitions. The most significant change, however,

5460-447: The instant at the beginning of a calendar day; and "24:00:00" to refer to midnight corresponding to the instant at the end of a calendar day. ISO 8601-1:2019 as originally published removed "24:00:00" as a representation for the end of day although it had been permitted in earlier versions of the standard. A decimal fraction may be added to the lowest order time element present in any of these representations. A decimal mark , either

5551-457: The intercalary month with an intercalary day to be inserted within February every four years. This produced a noticeably more accurate calendar, but it had an average year length of 365 days and six hours (365.25 days), which had the effect of adding about three-quarters of an hour every four years. The effect accumulated from inception in 45 BC until by the 16th century the northward equinox

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5642-409: The introduction of the Gregorian calendar ) are not automatically allowed by the standard. Instead, the standard states that "values in the range [0000] through [1582] shall only be used by mutual agreement of the partners in information interchange". To represent years before 0000 or after 9999 , the standard also permits the expansion of the year representation but only by prior agreement between

5733-513: The last week of its year. The week number can be described by counting the Thursdays: week 12 contains the 12th Thursday of the year. The ISO week-numbering year starts at the first day (Monday) of week 01 and ends at the Sunday before the new ISO year (hence without overlap or gap). It consists of 52 or 53 full weeks. The first ISO week of a year may have up to three days that are actually in

5824-588: The legacy one, i.e. compatible with it, a reform of the local calendar system is possible without disruption. Examples of this include the changes to the Chinese calendar due to problems between regions in China and practical changes in religious calendars such as the Islamic calendar , where the start of the month is now derived from astronomical data rather than sightings by religious leaders. Some design changes, however, will yield date identifiers different from

5915-509: The months inherited from the old Roman calendar as reformed by Julius Caesar do not follow any apparent logic systematically. Many reform proposals seek to make the pattern more uniform. When keeping the traditional dozen months and the close approximation of a solar year, this usually yields four equal quarters of three months each where one month is longer than the other two. World Calendar and Hanke–Henry Permanent Calendar follow this with 31:30:30 and 30:30:31 days per month, respectively. On

6006-400: The new year always begins within three days of the Gregorian new year. The key difference between Irv Bromberg's calendar proposal Symmetry010 and Hanke/Henry’s is the pattern of month lengths, the former putting the longer month in the middle of each quarter (30:31:30). The more ambitious Symmetry454 furthermore has every month consist of exactly 4 or 5 weeks (28:35:28). Both proposals start

6097-418: The next, thereby driving the cycles out of synchronization. A typical solution to force synchronization is intercalation . This means occasionally adding an extra day, week, or month into the cycle. An alternative approach is to ignore the mismatch and simply let the cycles continue to drift apart. The general approaches include: An obvious disadvantage of the lunisolar method of inserting a whole extra month

6188-471: The official Hanke-Henry calendar app, with capabilities to convert between Gregorian and Hanke-Henry Calendars, in order to facilitate transition to the Hanke-Henry system. It is no longer available as of March, 2018. The key difference between Robert McClenon's calendar proposal and Henry's modification is that the former has a simple rule for determining which years have a leap week. This rule resembles

6279-419: The other hand, Symmetry454 uses 4:5:4 weeks per month. They all result in 364 systematically distributed days and hence have to add either one extra and one leap day or a leap week. Some calendar reformers seek to equalize the length of each month in the year. This is often accomplished by creating a calendar that has 13 months of 4 weeks (28 days) each, making 364 days. The earliest known proposal of this type

6370-626: The previous design for some days, often in the distant past or future. The calendar system must clarify whether dates are changed to the new design retroactively (using a proleptic calendar ) or whether the design in use then and there shall be respected. Calendar schisms happen if not all cultures that adopted a common calendar system before perform a calendar reform at the same time. If date identifiers are similar but different, confusion and mistakes are unavoidable. Most calendars have several rules which could be altered by reform: Historically, most calendar reforms have been made in order to synchronize

6461-426: The same structure given above, but inserted its leap week named "Newton" between June and July in the middle of the year. The leap rule was chosen to match the ISO week leap rule, to minimise the variation in the start of the year relative to the Gregorian calendar, whereas Robert McClenon originally proposed a simple leap rule which would result in larger astronomic variance: Years whose numbers are divisible by 5 had

6552-426: The sender and the receiver. An expanded year representation [± Y YYYY] must have an agreed-upon number of extra year digits beyond the four-digit minimum, and it must be prefixed with a + or − sign instead of the more common AD/BC (or CE/BCE ) notation; by convention 1 BC is labelled +0000 , 2 BC is labeled −0001, and so on. Calendar date representations are in the form shown in the adjacent box. [YYYY] indicates

6643-399: The seven-day week and caused some religious groups to strongly oppose adoption of the World Calendar. Such concerns helped prevent the World Calendar from being adopted. Supporters of the World Calendar, however, argue that the religious groups' opposition overlooked every individual's right to celebrate these holidays as extra days of worship, or Sabbaths . This option, they reason, maintained

6734-539: The seven-day worship cycle for those who share that concern, while allowing benefits of a perennial calendar to be shared by all. Some calendar reform ideas, such as the Pax Calendar , Symmetry454 calendar and the Hanke–Henry Permanent Calendar , were created to solve this problem by having years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week. The 53-week calendar, used in government and in business for fiscal years ,

6825-456: The standard's notation. If the time is in UTC , add a Z directly after the time without a space. Z is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "T0930Z". "14:45:15 UTC" would be "14:45:15Z" or "T144515Z". The Z suffix in the ISO 8601 time representation is sometimes referred to as "Zulu time" or "Zulu meridian" because the same letter

6916-534: The standard. The first edition of the ISO 8601 standard was published as ISO 8601:1988 in 1988. It unified and replaced a number of older ISO standards on various aspects of date and time notation: ISO 2014 , ISO 2015 , ISO 2711 , ISO 3307 , and ISO 4031 . It has been superseded by a second edition ISO 8601:2000 in 2000, by a third edition ISO 8601:2004 published on 1 December 2004, and withdrawn and revised by ISO 8601-1:2019 and ISO 8601-2:2019 on 25 February 2019. ISO 8601

7007-548: The traditional numbers of days per week and months per year respectively. The nearby numbers 360, 364 and 366 are divisible in better ways. There are also lunar-centric proposals. Many calendar reforms have offered solutions to make the Gregorian calendar perennial. These reforms would make it easy to work out the day of the week of a particular date , and would make changing calendars each year unnecessary. There are, roughly speaking, two options to achieve this goal: leap week calendars and intercalary days . Leap week calendars add

7098-431: The truncation "--04-05" to mean "April 5" but the 2004 version does not allow omitting the year when a month is present. Examples: Week date representations are in the formats as shown in the adjacent box. [YYYY] indicates the ISO week-numbering year which is slightly different from the traditional Gregorian calendar year (see below). [Www] is the week number prefixed by the letter W , from W01 through W53. [D]

7189-438: The use of "−00" with the same denotation as "+00" but a differing connotation: an unknown UTC offset . To represent a negative offset, ISO 8601 specifies using a minus sign ( − ) . If the interchange character set is limited and does not have a minus sign character, then the hyphen-minus should be used, ( - ). ASCII does not have a minus sign, so its hyphen-minus character (code 45 10 ) would be used. If

7280-518: The week on Monday and are intended to be used with a different leap rule, resulting in a 293-year leap cycle. Other proposals, like the Pax Calendar from 1930 and the International Fixed Calendar popularized by Cotsworth and Eastman, feature a perennial calendar with 13 months of 28 days each. The former also has a leap week whereas the latter has one day at the end of each year belonging to no month or week and another in leap years. Calendar reform Calendar reform or calendrical reform

7371-509: The weekly cycle intact, and therefore respects the Fourth Commandment ( Remember the sabbath day, to keep it holy ) of Judaism and Christianity. In 2004, Richard Conn Henry , a professor of astronomy at Johns Hopkins University , proposed the adoption of a calendar known as Common-Civil-Calendar-and-Time (CCC&T), which he described as a modification to a proposal by Robert McClenon. Henry's original version had essentially

7462-425: The world. There is also an international standard describing the calendar, ISO 8601 , with some differences from traditional conceptions in many cultures. Since the papal reform in 1582, several proposals have been offered to make the Gregorian calendar more useful or regular. Very few reforms have gained official acceptance. The rather different decimal French Republican Calendar was one such official reform, but

7553-503: The year, when needed, whereas its successor, the Hanke–Henry Permanent Calendar, moves the extra week to the end of the year. In the World Season Calendar , months are discarded altogether; instead, the year is divided into four seasons of 13 weeks each. An extra day (two days during leap year) is added to the calendar that is not assigned a day of the week in order to perennialize the calendar. The same calendar of 91 days

7644-511: The years entirely, it would require a few thousand years to accumulate a day. So that the northward equinox would have the same date in the new Gregorian calendar as it had when the Council of Nicaea made recommendations in AD ;325 ( March 21 ), ten days were dropped so that October 5 became October 15 in 1582. This reform took a few centuries to spread through the nations that used

7735-568: Was a reform of the preceding lunisolar calendar which completely divorced it from the solar year. Another reform was performed in Seljuk Persia by Omar Khayyam and others, developing the precisely computed Jalali calendar . When Julius Caesar took power in Rome, the Roman calendar had ceased to reflect the year accurately. The Julian reform made 46 BC 445 days long and replaced

7826-570: Was abolished twelve years later by Napoleon . After World War II , the newly formed United Nations continued efforts of its predecessor, the League of Nations , to establish the proposed World Calendar but postponed the issue after a veto from the government of the United States , which was mainly based upon concerns of religious groups about the proposed days that would be outside the seven-day week cycle ("blank days") and thus disrupt having

7917-470: Was falling on March 10 or 11. Under Pope Gregory XIII , the leap year rule was altered : only centennial years evenly divisible by 400 are leap years. Thus, the years 1600, 2000, 2400 and 2800 are leap years , while 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 are common years despite being divisible by 4. This rule makes the mean year 365.2425 days (365 d, 5 h, 49 min, 12 s) long. While this does not synchronize

8008-440: Was prepared by, and is under the direct responsibility of, ISO Technical Committee TC 154. ISO 2014, though superseded, is the standard that originally introduced the all-numeric date notation in most-to-least-significant order [YYYY]-[MM]-[DD] . The ISO week numbering system was introduced in ISO 2015, and the identification of days by ordinal dates was originally defined in ISO 2711. Issued in February 2019,

8099-425: Was removed in ISO 8601-1:2019). However, ISO calendar dates before the convention are still compatible with the Gregorian calendar all the way back to the official introduction of the Gregorian calendar on 15 October 1582. Earlier dates, in the proleptic Gregorian calendar , may be used by mutual agreement of the partners exchanging information. The standard states that every date must be consecutive, so usage of

8190-545: Was the Georgian Calendar (1745) by Rev. Hugh Jones . The Positivist calendar (1849), created by Auguste Comte , was based on a 364-day year which included one or two "blank" days. Each of the 13 months had 28 days and exactly four weeks, and each started on a Monday. The International Fixed Calendar is a more modern descendant of this calendar: invented by Moses B. Cotsworth and financially backed by George Eastman . Around 1930, one James Colligan invented

8281-523: Was the reintroduction of the "24:00:00" format to refer to the instant at the end of a calendar day. The standard uses the Gregorian calendar , which "serves as an international standard for civil use". ISO 8601:2004 fixes a reference calendar date to the Gregorian calendar of 20 May 1875 as the date the Convention du Mètre ( Metre Convention ) was signed in Paris (the explicit reference date

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