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Julian Day and Date Calculator

Change the date to calculate the Julian Day Number (JD#) and day of the week, or vice versa. Be certain to a enter positive year (there are no negative years), then specify A.D. or B.C.; remember, there was no year 0 (zero) so 0 is not a valid year.

The default date is today's date

In the AutoSelect mode (the default mode), the civil calendar changes from Julian to Gregorian between October 4/15, 1582. For Julian civil dates after that date, as in England and colonies until 1753, select Use Julian Only. A Julian date is valid from January 1, 4713 B.C. to present. For Financial dates and legal dates, days between dates, days before a date or days after a date, day of the week for a given date in history, involving the last 2 centuries, use the AutoSelect mode. Be sure to see the information below the calculator for general information about the Julian and Gregorian (accepted as our current civil calendar) calendars. In this calculator, we have avoided the "time of day" factor. It is briefly discussed below. If you have the need for that, you can use our calendar date to Julian date and time calculator or our calendar date and time From Julian date calculator.

Julian Day Calculator

Julian Day Number(JD#):
Select Calendar   AutoSelect (Julian / Gregorian) Use Julian Only

How many days away...

This additional calculation puts a number of days between date one and date two. You can add a number in the field, or change the dates to see how many days between the two.
If this doesn't make sense to you and you are wanting to know the number of days between two different dates, we suggest that you use our simplified Days Between Dates Calculator.

Date 1 is days earlier than date 2

Julian Day Number (JD#):
Select Calendar   AutoSelect (Julian / Gregorian) Use Julian Only

Julian Day Information

In astronomy, a JD (Julian Date) is defined as the contiguous count of days from January 1, 4713 B.C., Greenwich Mean Noon (equal to zero hours UTC). The fraction of each day is represented as a decimal number. Hence noon (GMT) on January 2, 4713 B.C. would have JD# (Julian Date number) 1.00000, 6.00 pm GMT on the same day would have JD# 1.25000 and 6.00 hours Coordinated Universal Time (UCT) would have JD# 2443509.75.

Why was January 1, 4713 B.C. chosen as the starting point? According to most knowledgeable sources on the subject, it is the most recent day (certainly that I can remember...) in which the year began on a Sunday with a full moon. Also day 1 was chosen as January 1, 4713 B.C. because the Julian Calendar, the Lunar Calendar and the Roman Tax Calendar all coincided. This happens every 7,980 years, so the next coincidence will be in 3267 A.D. I'll let you know if it actually happens that way next time...

Updated: 6/10/11

Calendar Date to Julian Date and Time Calculator
Calendar Date and Time From Julian Date Calculator

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