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Julian Date Calculator

Enter a calendar date to get its Julian day number.

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Formula

For year y, month m, day d (Gregorian):

  • a = floor((14 − m) ÷ 12)
  • Y = y + 4800 − a, M = m + 12a − 3
  • JDN = d + floor((153M + 2) ÷ 5) + 365Y + floor(Y ÷ 4) − floor(Y ÷ 100) + floor(Y ÷ 400) − 32045

Julian date at midnight = JDN − 0.5. Modified Julian date = JDN − 2400001.

Worked example

1 January 2000: a = 1, Y = 6799, M = 10. JDN = 1 + 306 + 2,481,635 + 1,699 − 67 + 16 − 32,045 = 2,451,545. The Julian date at 00:00 is 2,451,544.5 and the modified Julian date is 51,544.

Frequently asked questions

What is a Julian day number?

It is a continuous count of days used in astronomy, starting from 1 January 4713 BC. 1 January 2000 is Julian day number 2,451,545.

Why does the Julian date at 00:00 end in .5?

Julian days start at noon. Midnight is half a day earlier, so 1 January 2000 at 00:00 is JD 2,451,544.5.

What is the modified Julian date?

It is the Julian date minus 2,400,000.5, which makes the count start at midnight on 17 November 1858. 1 January 2000 is MJD 51,544.

Is this the same as the Julian calendar?

No. The Julian day count is separate from the old Julian calendar. This calculator reads your date as an ordinary Gregorian calendar date.

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