How to Calculate Julian Dates

You can calculate the Julian date of any day of our modern calendar -- also known as the Gregorian calendar -- with a little math.

Instructions

  • Step 1: Add 1 to the month number, and then multiply the result by 30.601. Set the value equal to F.
  • Step 2: Add up the day of the month, C, E, and F. Then, subtract 1,524.5. This is the Julian Day -- or Julian Day Number, for the date in question.
  • FACT: The Julian Day Count used in astronomy is a count of days from November 14, 4714 BCE. The concept was developed in 1583 CE.
  • Step 3: Add 4,716 to the original Gregorian year. Then, multiply the result by 365.25. Set this result equal to the variable E.
  • TIP: Drop the fraction from all results of all multiplications and divisions. Don't round up or down, just drop the fraction.
  • TIP: If the month is January or February, subtract 1 from the year to get a new year, and add 12 to the month to get a new month number. For example, if the month is January, and the date is 2010, change the year to 2009 and add 12 to 1 to get a value of M equal to 13.
  • Step 4: Perform 3 computations: Calculate the variable A which equals the Gregorian year divided by 100. Then calculate B, which equals A divided by 4. Finally, calculate C, which equals 2 minus A plus B.
  • Step 5: Write the Gregorian date out as year (Y), month (M) using numbers from 1 to 12, and day (D) of the month.

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