Welcome to calendar converter! This page allows you to interconvert dates in a variety of calendars, both civil and computer-related. All calculations are done in JavaScript executed in your own browser. To use the page, your browser must support JavaScript and you must not have disabled execution of that language. Let's see...

If the box above says "Your browser supports JavaScript", you're in business; simply enter a date in any of the boxes below and press the "Calculate" button to show that date in all of the other calendars.

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 *Julian era*,
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 Cćsar 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 when divided by 4 a remainder of 3 results.) 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.

The Hebrew (or Jewish) calendar attempts to
simultaneously maintain alignment between the months and the seasons and
synchronise months with the Moon--it is thus deemed a "*luni-solar
calendar*". In addition, there are constraints on which days of the week on
which a year can begin and to shift otherwise required extra days to prior years
to keep the length of the year within the prescribed bounds. This isn't easy,
and the computations required are correspondingly intricate.

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 Hegirć*"--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.

Cycle | Composed of | Total Days | Years (approx.) |
---|---|---|---|

kin
| 1 | ||

uinal
| 20 kin | 20 | |

tun
| 18 uinal | 360 | 0.986 |

katun
| 20 tun | 7200 | 19.7 |

baktun
| 20 katun | 144,000 | 394.3 |

pictun
| 20 baktun | 2,880,000 | 7,885 |

calabtun
| 20 piktun | 57,600,000 | 157,704 |

kinchiltun
| 20 calabtun | 1,152,000,000 | 3,154,071 |

alautun
| 20 kinchiltun | 23,040,000,000 | 63,081,429 |

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:

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 (19˛) 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 31 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 2000.

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.

You can download the ISO 8601
standard from the ISO Web site; to read this PDF document you'll need Adobe Acrobat Reader, which is
available as a free download from Adobe's site.

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.

The machines on which Unix was developed and initially 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 the original C
implementation).

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. Modern Unix and C implementations 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.

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 remedied.

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 *almost forever* to add or subtract
four in order to make everything seamlessly interchangeable.

Macintosh Excel day numbers are valid only between 0 (January 1, 1904) and
2957003 (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. 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 63918 (December 31, 2078); I do not
know in which release of Excel this limitation was remedied.