Julian Date Converter
Calendar Date to Julian Date
Date to Calendar Date
Julian dates (abbreviated JD) are simply a continuous count of days and
fractions since noon Universal Time on January 1, 4713 BC (on the Julian
calendar). Almost 2.5 million days have transpired since this date. Julian
dates are widely used as time variables within astronomical software.
Typically, a 64-bit floating point (double precision) variable can represent
an epoch expressed as a Julian date to about 20 microsecond precision. Note
that the time scale that is the basis for Julian dates is
Universal Time (UT1),
and that 0h UT1 corresponds to a Julian date fraction of 0.5.
It is assumed that 7-day weeks have formed an uninterrupted sequence since
ancient times. Thus, the day of the week can be obtained from the remainder
of the division of the Julian date by 7.
Calendar dates — year, month, and day — are more problematic.
Various calendar systems have been in use at different times and places
around the world. This application deals with only two: the Gregorian
calendar, now used universally for civil purposes, and the Julian calendar,
its predecessor in the western world. As used here, the two calendars have
identical month names and number of days in each month, and 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 century
years not exactly divisible by 400.
This application assumes that the changeover from the Julian calendar to the
Gregorian calendar occurred in October of 1582, according to the scheme
instituted by Pope Gregory XIII. Specifically, for dates on or before 4 October
1582, the Julian calendar is used; for dates on or after 15 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: 4 October 1582 (Julian)
is a Thursday, which begins at JD 2299159.5; and 15 October 1582 (Gregorian) is
a Friday, which begins at JD 2299160.5. The omission of ten days of calendar
dates was necessitated by the astronomical error built up by the Julian
calendar over its many centuries of use, due to its too-frequent leap years.
The changeover to the Gregorian calendar system occurred as described above only
in Roman Catholic countries. However, adoption of the Gregorian calendar in the
rest of the world progressed slowly. For example, for England and its colonies,
the change did not occur until September 1752. (The Unix cal command for
systems manufactured in the U.S. reflects the 1752 changeover.)
of when certain countries switched to the Gregorian calendar
, see Claus
For further information on calendars, see Richards, E.G. 2012,
"Calendars," from the
Supplement to the Astronomical Almanac, 3rd edition, S.E Urban
and P.K. Seidelmann eds., (Mill Valley, CA: University Science Books),
Chapter 15, pp. 585-624.
The modified Julian date (MJD) is related to the Julian date (JD) by the formula:
MJD = JD - 2400000.5
MJD's are sometimes used for modern-era scientific data.
Application Programming Interface