Julian dates, stored as two numbers, from proleptic Gregorian and Julian calendars.




This module contains functions for converting between Julian dates and calendar dates.

A function for converting Gregorian calendar dates to Julian dates, and another function for converting Julian calendar dates to Julian dates are defined. Two functions for the reverse calculations are also defined.

Different regions of the world switched to Gregorian calendar from Julian calendar on different dates. Having separate functions for Julian and Gregorian calendars allow maximum flexibility in choosing the relevant calendar.

Julian dates are stored in two floating point numbers (double). Julian dates, and Modified Julian dates, are large numbers. If only one number is used, then the precision of the time stored is limited. Using two numbers, time can be split in a manner that will allow maximum precision. For example, the first number could be the Julian date for the beginning of a day and the second number could be the fractional day. Calculations that need the latter part can now work with maximum precision.

All the above functions are "proleptic". This means that they work for dates on which the concerned calendar is not valid. For example, Gregorian calendar was not used prior to around October 1582.

A function to test if a given Gregorian calendar year is a leap year is also defined.

Zero point of Modified Julian Date (MJD) and the MJD of 2000/1/1 12:00:00 are also given as module level constants.


Some examples are given below. For more information see http://oneau.wordpress.com/jdcal/.

Gregorian calendar:

>>> from jdcal import gcal2jd, jd2gcal
>>> gcal2jd(2000,1,1)
(2400000.5, 51544.0)
>>> 2400000.5 + 51544.0 + 0.5

>>> gcal2jd(2000,2,30)
(2400000.5, 51604.0)
>>> gcal2jd(2000,3,1)
(2400000.5, 51604.0)
>>> gcal2jd(2001,2,30)
(2400000.5, 51970.0)
>>> gcal2jd(2001,3,2)
(2400000.5, 51970.0)

>>> jd2gcal(*gcal2jd(2000,1,1))
(2000, 1, 1, 0.0)
>>> jd2gcal(*gcal2jd(1950,1,1))
(1950, 1, 1, 0.0)

>>> gcal2jd(2000,1,1)
(2400000.5, 51544.0)
>>> jd2gcal(2400000.5, 51544.0)
(2000, 1, 1, 0.0)
>>> jd2gcal(2400000.5, 51544.5)
(2000, 1, 1, 0.5)
>>> jd2gcal(2400000.5, 51544.245)
(2000, 1, 1, 0.24500000000261934)
>>> jd2gcal(2400000.5, 51544.1)
(2000, 1, 1, 0.099999999998544808)
>>> jd2gcal(2400000.5, 51544.75)
(2000, 1, 1, 0.75)

Julian calendar:

>>> jd2jcal(*jcal2jd(2000, 1, 1))
(2000, 1, 1, 0.0)
>>> jd2jcal(*jcal2jd(-4000, 10, 11))
(-4000, 10, 11, 0.0)

Gregorian leap year:

>>> from jdcal import is_leap
>>> is_leap(2000)
>>> is_leap(2100)

JD for zero point of MJD, and MJD for JD2000.0:

>>> from jdcal import MJD_0, MJD_JD2000
>>> print MJD_0
>>> print MJD_JD2000


The module can be installed using pip or easy_install:

$ pip install jdcal


$ easy_install jdcal

Tests are in test_jdcal.py.


  1. A good amount of the code is based on the excellent TPM C library by Jeffrey W. Percival.
  2. The inspiration to split Julian dates into two numbers came from the IAU SOFA C library. No code or algorithm from the SOFA library is used in jdcal.


Released under BSD; see LICENSE.txt.

For comments and suggestions, email to user prasanthhn in the gmail.com domain.