The Gregorian calendar was proclaimed by Pope Gregory XIII and took effect in most Catholic states in 1582, in which October 4, 1582 of the Julian calendar was followed by October 15 in the new calendar, correcting for the accumulated discrepancy between the Julian calendar and the equinox as of that date. When comparing historical dates, it's important to note that the Gregorian calendar, used universally today in Western countries and in international commerce, was adopted at different times by different countries. Britain and her colonies (including what is now the United States), did not switch to the Gregorian calendar until 1752, when Wednesday 2nd September in the Julian calendar dawned as Thursday the 14th in the Gregorian.
The Gregorian calendar is a minor correction to the Julian. In the Julian calendar every fourth year is a leap year in which February has 29, not 28 days, but in the Gregorian, years divisible by 100 are not leap years unless they are also divisible by 400. How prescient was Pope Gregory! Whatever the problems of Y2K, they won't include sloppy programming which assumes every year divisible by 4 is a leap year since 2000, unlike the previous and subsequent years divisible by 100, is a leap year. As in the Julian calendar, days are considered to begin at midnight.
The average length of a year in the Gregorian calendar is 365.2425 days compared to the actual solar tropical year (time from equinox to equinox) of 365.24219878 days, so the calendar accumulates one day of error with respect to the solar year about every 3300 years. As a purely solar calendar, no attempt is made to synchronise the start of months to the phases of the Moon.
While one can't properly speak of "Gregorian dates" prior to the adoption of the calendar in 1582, the calendar can be extrapolated to prior dates. In doing so, this implementation uses the convention that the year prior to year 1 is year 0. This differs from the Julian calendar in which there is no year 0--the year before year 1 in the Julian calendar is year -1. The date December 30th, 0 in the Gregorian calendar corresponds to January 1st, 1 in the Julian calendar.
A slight modification of the Gregorian calendar would make it even more precise. If you add the additional rule that years evenly divisible by 4000 are not leap years, you obtain an average solar year of 365.24225 days per year which, compared to the actual mean year of 365.24219878, is equivalent to an error of one day over a period of about 19,500 years; this is comparable to errors due to tidal braking of the rotation of the Earth.
Astronomers, unlike historians, frequently need to do arithmetic with
dates. For example: a double star goes into eclipse every 1583.6 days and
its last mid-eclipse was measured to be on October 17, 2003 at 21:17 UTC.
When is the next? Well, you could get out your calendar and count days,
but it's far easier to convert all the quantities in question to Julian
day numbers and simply add or subtract. Julian days simply enumerate the
days and fraction which have elapsed since the start of the
which is defined as beginning at noon on Monday, 1st January of year 4713
B.C.E. in the Julian calendar. This date is defined in terms of a cycle
of years, but has the additional advantage that all known historical astronomical
observations bear positive Julian day numbers, and periods can be determined
and events extrapolated by simple addition and subtraction. Julian dates
are a tad eccentric in starting at noon, but then so are astronomers (and
systems programmers!)--when you've become accustomed to rising after the
"crack of noon" and doing most of your work when the Sun is down, you appreciate
recording your results in a calendar where the date doesn't change in the
middle of your workday. But even the Julian day convention bears witness
to the eurocentrism of 19th century astronomy--noon at Greenwich is midnight
on the other side of the world. But the Julian day notation is so deeply
embedded in astronomy that it is unlikely to be displaced at any time in
the foreseeable future. It is an ideal system for storing dates in computer
programs, free of cultural bias and discontinuities at various dates, and
can be readily transformed into other calendar systems, as the source code
for this page illustrates. Use Julian days and fractions (stored in 64
bit or longer floating point numbers) in your programs, and be ready for
Y10K, Y100K, and Y1MM!
While any event in recorded human history can be written as a positive Julian day number, when working with contemporary events all those digits can be cumbersome. A Modified Julian Day (MJD) is created by subtracting 2400000.5 from a Julian day number, and thus represents the number of days elapsed since midnight (00:00) Universal Time on November 17, 1858. Modified Julian Days are widely used to specify the epoch in tables of orbital elements of artificial Earth satellites. Since no such objects existed prior to October 4, 1957, all satellite-related MJDs are positive.
The Julian calendar was proclaimed by Julius Cæsar in 46 B.C. and underwent several modifications before reaching its final form in 8 C.E. The Julian calendar differs from the Gregorian only in the determination of leap years, lacking the correction for years divisible by 100 and 400 in the Gregorian calendar. In the Julian calendar, any positive year is a leap year if divisible by 4. (Negative years are leap years if the absolute value divided by 4 yields a remainder of 1.) Days are considered to begin at midnight.
In the Julian calendar the average year has a length of 365.25 days.
compared to the actual solar tropical year of 365.24219878 days. The calendar
thus accumulates one day of error with respect to the solar year every
128 years. Being a purely solar calendar, no attempt is made to synchronise
the start of months to the phases of the Moon.
Years are classified as common (normal) or embolismic (leap) years which occur in a 19 year cycle in years 3, 6, 8, 11, 14, 17, and 19. In an embolismic (leap) year, an extra month of 29 days, "Veadar" or "Adar II", is added to the end of the year after the month "Adar", which is designated "Adar I" in such years. Further, years may be deficient, regular, or complete, having respectively 353, 354, or 355 days in a common year and 383, 384, or 385 days in embolismic years. Days are defined as beginning at sunset, and the calendar begins at sunset the night before Monday, October 7, 3761 B.C.E. in the Julian calendar, or Julian day 347995.5. Days are numbered with Sunday as day 1, through Saturday: day 7.
The average length of a month is 29.530594 days, extremely close to the mean synodic month (time from new Moon to next new Moon) of 29.530588 days. Such is the accuracy that more than 13,800 years elapse before a single day discrepancy between the calendar's average reckoning of the start of months and the mean time of the new Moon. Alignment with the solar year is better than the Julian calendar, but inferior to the Gregorian. The average length of a year is 365.2468 days compared to the actual solar tropical year (time from equinox to equinox) of 365.24219 days, so the calendar accumulates one day of error with respect to the solar year every 216 years.
The Islamic calendar is purely lunar and consists of twelve alternating months of 30 and 29 days, with the final 29 day month extended to 30 days during leap years. Leap years follow a 30 year cycle and occur in years 1, 5, 7, 10, 13, 16, 18, 21, 24, 26, and 29. Days are considered to begin at sunset. The calendar begins on Friday, July 16th, 622 C.E. in the Julian calendar, Julian day 1948439.5, the day of Muhammad's flight from Mecca to Medina, with sunset on the preceding day reckoned as the first day of the first month of year 1 A.H.--"Anno Hegiræ"--the Arabic word for "separate" or "go away". Weeks begin on Sunday, and the names for the days are just their numbers: Sunday is the first day and Saturday the seventh.
Each cycle of 30 years thus contains 19 normal years of 354 days and 11 leap years of 355, so the average length of a year is therefore ((19 × 354) + (11 × 355)) / 30 = 354.365... days, with a mean length of month of 1/12 this figure, or 29.53055... days, which closely approximates the mean synodic month (time from new Moon to next new Moon) of 29.530588 days, with the calendar only slipping one day with respect to the Moon every 2525 years. Since the calendar is fixed to the Moon, not the solar year, the months shift with respect to the seasons, with each month beginning about 11 days earlier in each successive solar year.
The calendar presented here is the most commonly used civil calendar in the Islamic world; for religious purposes months are defined to start with the first observation of the crescent of the new Moon.
The modern Persian calendar was adopted in 1925, supplanting (while retaining the month names of) a traditional calendar dating from the eleventh century. The calendar consists of 12 months, the first six of which are 31 days, the next five 30 days, and the final month 29 days in a normal year and 30 days in a leap year.
As one of the few calendars designed in the era of accurate positional astronomy, the Persian calendar uses a very complex leap year structure which makes it the most accurate solar calendar in use today. Years are grouped into cycles which begin with four normal years after which every fourth subsequent year in the cycle is a leap year. Cycles are grouped into grand cycles of either 128 years (composed of cycles of 29, 33, 33, and 33 years) or 132 years, containing cycles of of 29, 33, 33, and 37 years. A great grand cycle is composed of 21 consecutive 128 year grand cycles and a final 132 grand cycle, for a total of 2820 years. The pattern of normal and leap years which began in 1925 will not repeat until the year 4745!
Each 2820 year great grand cycle contains 2137 normal years of 365 days and 683 leap years of 366 days, with the average year length over the great grand cycle of 365.24219852. So close is this to the actual solar tropical year of 365.24219878 days that the Persian calendar accumulates an error of one day only every 3.8 million years. As a purely solar calendar, months are not synchronised with the phases of the Moon.
The Mayans employed three calendars, all organised as hierarchies of
cycles of days of various lengths. The Long Count was the principal
calendar for historical purposes, the Haab was used as the civil
calendar, while the Tzolkin was the religious calendar. All of the
Mayan calendars are based on serial counting of days without means for
synchronising the calendar to the Sun or Moon, although the Long Count
and Haab calendars contain cycles of 360 and 365 days, respectively, which
are roughly comparable to the solar year. Based purely on counting days,
the Long Count more closely resembles the Julian Day system and contemporary
computer representations of date and time than other calendars devised
in antiquity. Also distinctly modern in appearance is that days and cycles
count from zero, not one as in most other calendars, which simplifies the
computation of dates, and that numbers as opposed to names were used for
all of the cycles.
The Long Count calendar is organised into the hierarchy of cycles shown at the right. Each of the cycles is composed of 20 of the next shorter cycle with the exception of the tun, which consists of 18 uinal of 20 days each. This results in a tun of 360 days, which maintains approximate alignment with the solar year over modest intervals--the calendar comes undone from the Sun 5 days every tun.
The Mayans believed at at the conclusion of each pictun cycle of about 7,885 years the universe is destroyed and re-created. Those with apocalyptic inclinations will be relieved to observe that the present cycle will not end until Columbus Day, October 12, 4772 in the Gregorian calendar. Speaking of apocalyptic events, it's amusing to observe that the longest of the cycles in the Mayan calendar, alautun, about 63 million years, is comparable to the 65 million years since the impact which brought down the curtain on the dinosaurs--an impact which occurred near the Yucatan peninsula where, almost an alautun later, the Mayan civilisation flourished. If the universe is going to be destroyed and the end of the current pictun, there's no point in writing dates using the longer cycles, so we dispense with them here.
Dates in the Long Count calendar are written, by convention, as:
baktun . katun . tun . uinal . kin
and thus resemble present-day Internet IP addresses!
For civil purposes the Mayans used the Haab calendar in which the year was divided into 18 named periods of 20 days each, followed by five Uayeb days not considered part of any period. Dates in this calendar are written as a day number (0 to 19 for regular periods and 0 to 4 for the days of Uayeb) followed by the name of the period. This calendar has no concept of year numbers; it simply repeats at the end of the complete 365 day cycle. Consequently, it is not possible, given a date in the Haab calendar, to determine the Long Count or year in other calendars. The 365 day cycle provides better alignment with the solar year than the 360 day tun of the Long Count but, lacking a leap year mechanism, the Haab calendar shifted one day with respect to the seasons about every four years.
The Mayan religion employed the Tzolkin calendar, composed of 20 named periods of 13 days. Unlike the Haab calendar, in which the day numbers increment until the end of the period, at which time the next period name is used and the day count reset to 0, the names and numbers in the Tzolkin calendar advance in parallel. On each successive day, the day number is incremented by 1, being reset to 0 upon reaching 13, and the next in the cycle of twenty names is affixed to it. Since 13 does not evenly divide 20, there are thus a total of 260 day number and period names before the calendar repeats. As with the Haab calendar, cycles are not counted and one cannot, therefore, convert a Tzolkin date into a unique date in other calendars. The 260 day cycle formed the basis for Mayan religious events and has no relation to the solar year or lunar month.
The Mayans frequently specified dates using both the Haab and Tzolkin calendars; dates of this form repeat only every 52 solar years.
The Bahá'í calendar is a solar calendar organised as a hierarchy of cycles, each of length 19, commemorating the 19 year period between the 1844 proclamation of the Báb in Shiraz and the revelation by Bahá'u'lláh in 1863. Days are named in a cycle of 19 names. Nineteen of these cycles of 19 days, usually called "months" even though they have nothing whatsoever to do with the Moon, make up a year, with a period between the 18th and 19th months referred to as Ayyám-i-Há not considered part of any month; this period is four days in normal years and five days in leap years. The rule for leap years is identical to that of the Gregorian calendar, so the Bahá'í calendar shares its accuracy and remains synchronised. The same cycle of 19 names is used for days and months.
The year begins at the equinox, March 21, the Feast of Naw-Rúz; days begin at sunset. Years have their own cycle of 19 names, called the Váhid. Successive cycles of 19 years are numbered, with cycle 1 commencing on March 21, 1844, the year in which the Báb announced his prophecy. Cycles, in turn, are assembled into Kull-I-Shay super-cycles of 361 (19²) years. The first Kull-I-Shay will not end until Gregorian calendar year 2205. A week of seven days is superimposed on the calendar, with the week considered to begin on Saturday. Confusingly, three of the names of weekdays are identical to names in the 19 name cycles for days and months.
A bewildering variety of calendars have been and continue to be used in the Indian subcontinent. In 1957 the Indian government's Calendar Reform Committee adopted the National Calendar of India for civil purposes and, in addition, defined guidelines to standardise computation of the religious calendar, which is based on astronomical observations. The civil calendar is used throughout India today for administrative purposes, but a variety of religious calendars remain in use. We present the civil calendar here.
The National Calendar of India is composed of 12 months. The first month, Caitra, is 30 days in normal and 31 days in leap years. This is followed by five consecutive 31 day months, then six 30 day months. Leap years in the Indian calendar occur in the same years as as in the Gregorian calendar; the two calendars thus have identical accuracy and remain synchronised.
Years in the Indian calendar are counted from the start of the Saka Era, the equinox of March 22nd of year 79 in the Gregorian calendar, designated day 1 of month Caitra of year 1 in the Saka Era. The calendar was officially adopted on 1 Caitra, 1879 Saka Era, or March 22nd, 1957 Gregorian. Since year 1 of the Indian calendar differs from year 1 of the Gregorian, to determine whether a year in the Indian calendar is a leap year, add 78 to the year of the Saka era then apply the Gregorian calendar rule to the sum.
The French Republican calendar was adopted by a decree of La Convention Nationale on Gregorian date October 5, 1793 and went into effect the following November 24th, on which day Fabre d'Églantine proposed to the Convention the names for the months. It incarnates the revolutionary spirit of "Out with the old! In with the relentlessly rational!" which later gave rise in 1795 to the metric system of weights and measures which has proven more durable than the Republican calendar.
The calendar consists of 12 months of 30 days each, followed by a five- or six-day holiday period, the jours complémentaires or sans-culottides. Months are grouped into four seasons; the three months of each season end with the same letters and rhyme with one another. The calendar begins on Gregorian date September 22nd, 1792, the September equinox and date of the founding of the First Republic. This day is designated the first day of the month of Vendémiaire in year 1 of the Republic. Subsequent years begin on the day in which the September equinox occurs as reckoned at the Paris meridian. Days begin at true solar midnight. Whether the sans-culottides period contains five or six days depends on the actual date of the equinox. Consequently, there is no leap year rule per se: 366 day years do not recur in a regular pattern but instead follow the dictates of astronomy. The calendar therefore stays perfectly aligned with the seasons. No attempt is made to synchronise months with the phases of the Moon.
The Republican calendar is rare in that it has no concept of a seven day week. Each thirty day month is divided into three décades of ten days each, the last of which, décadi, was the day of rest. (The word "décade" may confuse English speakers; the French noun denoting ten years is "décennie".) The names of days in the décade are derived from their number in the ten day sequence. The five or six days of the sans-culottides do not bear the names of the décade. Instead, each of these holidays commemorates an aspect of the republican spirit. The last, jour de la Révolution, occurs only in years of 366 days.
Napoléon abolished the Republican calendar in favour of the Gregorian on January 1st, 1806. Thus France, one of the first countries to adopt the Gregorian calendar (in December 1582), became the only country to subsequently abandon and then re-adopt it. During the period of the Paris Commune uprising in 1871 the Republican calendar was again briefly used.
The original decree which established the Republican calendar contained a contradiction: it defined the year as starting on the day of the true autumnal equinox in Paris, but further prescribed a four year cycle called la Franciade, the fourth year of which would end with le jour de la Révolution and hence contain 366 days. These two specifications are incompatible, as 366 day years defined by the equinox do not recur on a regular four year schedule. This problem was recognised shortly after the calendar was proclaimed, but the calendar was abandoned five years before the first conflict would have occurred and the issue was never formally resolved. Here we assume the equinox rule prevails, as a rigid four year cycle would be no more accurate than the Julian calendar, which couldn't possibly be the intent of its enlightened Republican designers.
The International Standards Organisation (ISO) issued Standard ISO 8601,
"Representation of Dates" in 1988, superseding the earlier ISO 2015. The
bulk of the standard consists of standards for representing dates in the
Gregorian calendar including the highly recommended "YYYY-MM-DD"
form which is unambiguous, free of cultural bias, can be sorted into order
without rearrangement, and is Y9K compliant. In addition, ISO 8601 formally
defines the "calendar week" often encountered in commercial transactions
in Europe. The first calendar week of a year: week 1, is that week which
contains the first Thursday of the year (or, equivalently, the week which
includes January 4th of the year; the first day of that week is the previous
Monday). The last week: week 52 or 53 depending on the date of Monday in
the first week, is that which contains December 28th of the year. The first
ISO calendar week of a given year starts with a Monday which can be as
early as December 29th of the previous year or as late as January 4th of
the present; the last calendar week can end as late as Sunday, January
3rd of the subsequent year. ISO 8601 dates in year, week, and day form
are written with a "W" preceding the week number, which bears a leading
zero if less than 10, for example February 29th, 2000 is written as 2000-02-29
in year, month, day format and 2000-W09-2 in year, week, day form; since
the day number can never exceed 7, only a single digit is required. The
hyphens may be elided for brevity and the day number omitted if not required.
You will frequently see date of manufacture codes such as "00W09" stamped
on products; this is an abbreviation of 2000-W09, the ninth week of year
In solar calendars such as the Gregorian, only days and years have physical significance: days are defined by the rotation of the Earth, and years by its orbit about the Sun. Months, decoupled from the phases of the Moon, are but a memory of forgotten lunar calendars, while weeks of seven days are entirely a social construct--while most calendars in use today adopt a cycle of seven day names or numbers, calendars with name cycles ranging from four to sixty days have been used by other cultures in history.
ISO 8601 permits us to jettison the historical and cultural baggage of weeks and months and express a date simply by the year and day number within that year, ranging from 001 for January 1st through 365 (366 in a leap year) for December 31st. This format makes it easy to do arithmetic with dates within a year, and only slightly more complicated for periods which span year boundaries. You'll see this representation used in project planning and for specifying delivery dates. ISO dates in this form are written as "YYYY-DDD", for example 2000-060 for February 29th, 2000; leading zeroes are always written in the day number, but the hyphen may be omitted for brevity.
All ISO 8601 date formats have the advantages of being fixed length (at least until the Y10K crisis rolls around) and, when stored in a computer, of being sorted in date order by an alphanumeric sort of their textual representations. The ISO week and day and day of year calendars are derivative of the Gregorian calendar and share its accuracy.
Development of the Unix operating system began at Bell Laboratories in 1969 by Dennis Ritchie and Ken Thompson, with the first PDP-11 version becoming operational in February 1971. Unix wisely adopted the convention that all internal dates and times (for example, the time of creation and last modification of files) were kept in Universal Time, and converted to local time based on a per-user time zone specification. This far-sighted choice has made it vastly easier to integrate Unix systems into far-flung networks without a chaos of conflicting time settings.
Many machines on which Unix was initially widely deployed could not support arithmetic on integers longer than 32 bits without costly multiple-precision computation in software. The internal representation of time was therefore chosen to be the number of seconds elapsed since 00:00 Universal time on January 1, 1970 in the Gregorian calendar (Julian day 2440587.5), with time stored as a 32 bit signed integer (long in early C implementations).
The influence of Unix time representation has spread well beyond Unix since most C and C++ libraries on other systems provide Unix-compatible time and date functions. The major drawback of Unix time representation is that, if kept as a 32 bit signed quantity, on January 19, 2038 it will go negative, resulting in chaos in programs unprepared for this. Unix and C implementations wisely (for reasons described below) define the result of the time() function as type time_t, which leaves the door open for remediation (by changing the definition to a 64 bit integer, for example) before the clock ticks the dreaded doomsday second.
C compilers on Unix systems prior to 7th Edition lacked the 32-bit long type. On earlier systems time_t, the value returned by the time() function, was an array of two 16-bit ints which, concatenated, represented the 32-bit value. This is the reason why time() accepts a pointer argument to the result (prior to 7th Edition it returned a status, not the 32-bit time) and ctime() requires a pointer to its input argument. Thanks to Eric Allman (author of sendmail) for pointing out these historical nuggets.
Spreadsheet calculations frequently need to do arithmetic with date and time quantities--for example, calculating the interest on a loan with a given term. When Microsoft Excel was introduced for the PC Windows platform, it defined dates and times as "serial values", which express dates and times as the number of days elapsed since midnight on January 1, 1900 with time given as a fraction of a day. Midnight on January 1, 1900 is day 1.0 in this scheme. Time zone is unspecified in Excel dates, with the NOW() function returning whatever the computer's clock is set to--in most cases local time, so when combining data from machines in different time zones you usually need to add or subtract the bias, which can differ over the year due to observance of summer time. Here we assume Excel dates represent Universal (Greenwich Mean) time, since there isn't any other rational choice. But don't assume you can always get away with this.
You'd be entitled to think, therefore, that conversion back and forth between PC Excel serial values and Julian day numbers would simply be a matter of adding or subtracting the Julian day number of December 31, 1899 (since the PC Excel days are numbered from 1). But this is a Microsoft calendar, remember, so one must first look to make sure it doesn't contain one of those bonehead blunders characteristic of Microsoft. As is usually the case, one doesn't have to look very far. If you have a copy of PC Excel, fire it up, format a cell as containing a date, and type 60 into it: out pops "February 29, 1900". News apparently travels very slowly from Rome to Redmond--ever since Pope Gregory revised the calendar in 1582, years divisible by 100 have not been leap years, and consequently the year 1900 contained no February 29th. Due to this morsel of information having been lost somewhere between the Holy See and the Infernal Seattle monopoly, all Excel day numbers for days subsequent to February 28th, 1900 are one day greater than the actual day count from January 1, 1900. Further, note that any computation of the number of days in a period which begins in January or February 1900 and ends in a subsequent month will be off by one--the day count will be one greater than the actual number of days elapsed.
By the time the 1900 blunder was discovered, Excel users had created millions of spreadsheets containing incorrect day numbers, so Microsoft decided to leave the error in place rather than force users to convert their spreadsheets, and the error remains to this day. Note, however, that only 1900 is affected; while the first release of Excel probably also screwed up all years divisible by 100 and hence implemented a purely Julian calendar, contemporary versions do correctly count days in 2000 (which is a leap year, being divisible by 400), 2100, and subsequent end of century years.
PC Excel day numbers are valid only between 1 (January 1, 1900) and
2958465 (December 31, 9999). Although a serial day counting scheme has
no difficulty coping with arbitrary date ranges or days before the start
of the epoch (given sufficient precision in the representation of numbers),
Excel doesn't do so. Day 0 is deemed the idiotic January 0, 1900 (at least
in Excel 97), and negative days and those in Y10K and beyond are not handled
at all. Further, old versions of Excel did date arithmetic using 16 bit
quantities and did not support day numbers greater than 65380 (December
31, 2078); I do not know in which release of Excel this limitation was
Having saddled every PC Excel user with a defective date numbering scheme
wasn't enough for Microsoft--nothing ever is. Next, they proceeded to come
out with a Macintosh version of Excel which uses an entirely different
day numbering system based on the MacOS native time format which counts
seconds elapsed since January 1, 1904. To further obfuscate matters, on
the Macintosh they chose to number days from zero rather than 1, so midnight
on January 1, 1904 has serial value 0.0. By starting in 1904, they avoided
screwing up 1900 as they did on the PC. So now Excel users who interchange
data have to cope with two incompatible schemes for counting days, one
of which thinks 1900 was a leap year and the other which doesn't go back
that far. To compound the fun, you can now select either date system on
either platform, so you can't be certain dates are compatible even when
receiving data from another user with same kind of machine you're using.
I'm sure this was all done in the interest of the "efficiency" of which
Microsoft is so fond. As we all know, it would take a computer
forever to add or subtract four in order to make everything seamlessly
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