This utility is based on a calculator that was originally published by the Astronomical Applications Department of the US Naval Observatory. However, the contents of that site have since changed, principally to initialise the form to midnight on the date shown by their server clock, not that in your computer. Since this server is located in the USA, the initialised date might be different from the local date by one day. This calculator uses the original format to convert from calendar date and time to Julian Date, and vice versa. The direction of conversion is selected by the Calculation Type. This page and its associated JavaScript file have been extended by Steve Glennie-Smith to increase accuracy to the nearest 10ms, to show a 'ticking' UTC clock and the local timezone. Bug fixes are listed at the end.
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Current Universal Time:- |
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The form is initialised to the Universal Date and Time (to all intents and purposes, this is the same as Greenwich Mean Time, but not exactly the same - see differences), as determined by the clock in your computer at the time this page is entered (note: this is always to the nearest second). A conversion from your local timezone is applied. The clock above the form shows Current Universal time, and will keep 'ticking'. The time shown in the form will not 'tick' - this and the date are free for you to alter as inputs to the Julian Date calculator. The weekday field is read-only, as determined from the Julian Date. The timezone field is also read-only; it is fixed according to the local date and time when your session was started. This includes any correction for 'daylight saving' (or 'Summer Time').
CE and BCE designate "Common Era" and "Before Common
Era", often known as "AD" and "BC"
respectively. See notes below for the Use Year Zero option.
Julian Dates (abbreviated JD) are simply a contiguous count of days and
fractions since noon Universal Time on 1st January, 4713 BCE (on the Julian
calendar). Almost 2.5 million days have elapsed since that date.
Julian Dates are widely used as time variables in astronomical software.
Typically, a 64-bit floating point (double precision) variable can represent
an epoch expressed as a Julian Date to about 1 millisecond precision.
Note that the time scale that is the basis for Julian Dates is Universal Time,
and that 00:00h UTC always occurs at a Julian Date fraction of 0.5. The
distinction between Julian Date and Julian Day is that the
former is the whole number, including the fractional part, whereas the latter
is just the integer part, ie. the number of days since 'Day Zero'.
Calendar dates — year,
month, and day — are more of a problem. Various calendar systems
have been in use at different times and places around the world. This
calculator deals with only two: the Gregorian calendar, now used universally
for civil purposes, and the Julian calendar, its predecessor in most of the
western world. As used here, the two calendars have identical month
names and number of days in each month; they differ only in the rule for leap
years. The Julian calendar has a leap year every fourth year, while the
Gregorian calendar has a leap year every fourth year except centennial
years that are not exactly divisible by 400.
Although JD zero was 1st January 4713 BCE, this
calculator will allow negative Julian Dates, and years with up to six figures.
The following assumptions are made:
Weeks have always had seven days, named as now,
since time began. Thus, the day of the week can be obtained
from the remainder, after dividing the Julian Date (plus the 0.5 day
offset, but then expressed as an integer) by 7
The Julian calendar had no year 0. Therefore,
The Julian calendar is assumed always to have had
leap years. Since it had no year 0, years 1, 5, etc. BCE
are considered to be leap years by this calculator (if Use
Year Zero is unchecked). However, some research (see below)
indicates that before the introduction of the Julian calendar in 45 BCE,
there was no consistency in the number of days in each year; This calculator does not take
Leap
Seconds into account. Since 00:00h UTC always occurs at a Julian
Date fraction of 0.5, it is assumed that the fractional part (which
represents hours, minutes, seconds and parts thereof) of Julian Dates where
a leap second is inserted must be 'stretched' on such dates.
Therefore calculations made for such dates will be inaccurate by up to one
second. Since leap seconds are only inserted (or theoretically, they
could be deleted) at midnight UTC, they will always occur at the
middle of a Julian Day. (Note: the purpose of a leap second is to
correct for slight variations in the time it takes for the earth to rotate
on its axis. This has no bearing on the number of days it takes for
the earth to orbit the sun, or any correction for variations in this as
applied by the Gregorian calendar); The changeover from the Julian calendar to the
Gregorian calendar occurred in October of 1582, in accordance with a
Papal Bull issued by Pope Gregory XIII. Specifically, for dates on
or before 4th October 1582, the Julian calendar is used; for dates on or
after 15th October 1582, the Gregorian calendar is used. Thus, there
is a ten-day gap in calendar dates, but no discontinuity in Julian Dates
or days of the week: 4th October 1582 (Julian) was a Thursday, which began
at JD 2299159.5; and 15th October 1582 (Gregorian) was a Friday, which
began at JD 2299160.5. It was necessary to delete ten calendar dates
owing to the error accumulated by the Julian calendar: over many
centuries of use, there had been too many leap years; The changeover to the Gregorian calendar only occurred
as described above in Roman Catholic countries, however. Adoption of the
Gregorian calendar in the rest of the world progressed slowly. For
example, Great Britain and its colonies did not implement the change until
September 1752 (though there is some doubt as to when the change took place in
Scotland). [The UNIX CAL command reflects the 1752 changeover,
when it became necessary to delete eleven days, since the year 1700 had
also been taken as a leap year.]
The corrections applied by the Gregorian calendar give a
close average approximation (365.2425 days) to the time it takes for the earth
to orbit the sun. However, the actual time taken (averaged over the last
few centuries) is 365.242190 days, so a small error still accrues, though now
it is now a gain of 1 day in about 3220 years. Various extensions to the
100 and 400 year rules in the Gregorian calendar
have been proposed, but none have officially been adopted:
A 4000 year rule: Astronomer John Herschel
suggested that if the year is divisible by 4000 it is not a leap
year. This gives a better approximation by having 969 leap years in
every 4000 (not 970), giving an average year of 365.24225 days.
Although not the most accurate of the proposed corrections, it neatly falls
within the A 3200 year rule, similar to the 4000 year rule
above: This is much closer to addressing the 3220 year discrepancy, but
seems to have even less support outside the military. The average
corrected year would be 365.2421875 days. A 128 year rule: This does away with the 100 and
400 year rules altogether. The proposal is to drop February 29th for
years divisible by 128, commencing in 2048. It is as accurate as the
3200 rule and the correction is applied more evenly. It would have
worked retrospectively: 1920 would not have been a leap year but 1900
would have been. Likewise 1792 vs. 1800 and 1664 vs. 1700.
1536 and earlier were in the Julian era, and so wouldn't have been
affected. Although great for computer geeks and those who love
powers of 2, it would not be easy to handle by ordinary mortals.
Since I will be 100 in 2048, if I'm still around then, I could expect my
Royal Telegram a day earlier if this system is adopted in the mean time!
A 900 year rule: This has been proposed by the
Greek Orthodox Church. It does away with the 400-year rule and
instead makes years divided by 900 that leave a remainder of 200 or 600
become leap years. The average corrected year length would be
365.242222 days. Both this and the Gregorian system give 2000 and
2400 as leap years. Differences start in 2800, which would not be a
leap year in the Greek system, but 2900 would be. In my view it would be churlish to impose changes that would
take place so far ahead on future generations. Who knows - some idiot
might let off an atom bomb or the earth could be hit by a big meteorite, both
of which could affect the length of the year, and so blow all this theory out
of the water. However, Julian Dates will keep plodding along
regardless. Since the aim of this calculator is to give an accurate
correlation between Julian Dates and Calendar Dates way into the future, I have
concurred with Herschel and UNIX, and altered the original JavaScript to
include the 4000 year rule.
More information on when various countries changed can be found on the Calendar pages of this European History site. For another list of when certain countries switched to the Gregorian calendar, see section 2.2.4 of Claus Tøndering's Calendar FAQ.
More information on calendars and their histories can
be found in L.E. Doggett's
"Calendars"
chapter of the
Explanatory Supplement to the
Astronomical Almanac (ed. P.K. Seidelmann, 1992,
University Science
Books). Note: these are commercial booksellers' sites.
Note: there are a number of other Julian Date calculators on the web: a quick search on Google turned up this one, which has huge errors. Quickly playing with it, it became apparent that it only gives correct answers between 1st March 1800 and 28th February 2100. Apart from 1900, it makes no correction for the Gregorian calendar and, according to it, JD zero was BCE4712 January 13th. Beware!
Bug fixes:
Steve Glennie-Smith 21st November 2007