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How to Calculate the MJD

The Julian Day (JD) is a continuous count of days and fractions of days from noon GMT, January 1st, 4713 B.C. The date is a whole number followed by decimal representing the time of day. A "remainder" indicates the day of the week.
2455004.25 2 = Tuesday, June 22, 2009, 6:00 PM GMT
Astronomers can plot the exact date of historical astronomical events no matter what calendar was in use at the time.
The modified Julian Date (MJD), introduced by the Smithsonian Astronomical Observatory in 1957 so its computer could track Sputnik, begins at midnight rather than at noon and is calculated as follows:

Things You'll Need

Calculator Computer with Internet connection Pencil &paper

Instructions

- 1
  Calculate the Julian Day. There are many free Julian Day Calculators available with a simple Internet search. There is a good one on the Wise Observatory's website (see below) Simply enter the date you want to convert and hit the calculate button and you have the Julian Day.
- 2
  Subtract 2400000 from the Julian day. Since for almost the next three centuries since November 17, 1858 the Julian day number will start with 24, for simplicity the Modified Julian date, we drop the first two numbers by subtracting 2400000 and we drop the half day by starting the hours/minutes/seconds calculation at midnight rather than noon.
- 3
  Add the decimals to indicate the time by first converting local time to Greenwich Mean Time (GMT). There are lots of time zone conversion sites on the Internet that will do that.
- 4
  Divide the hour by 24. Use military time (3:00 pm = 15:00).
  For example if it is 21:51:35 PM Greenwich mean time, you'd divide the hour (21) by 24.
  You get: .875
- 5
  Divide the minutes (51 in this example) by 1440 (the number of minutes in a day).
  You get: .0354166666666.....
  Round the last repeating 6 to get 0.035417
- 6
  Divide the seconds (35) by 86400.
  You get: 0.000405092
- 7
  Add the three together: .875 + .035417 + .000405092
  The total is: 0.910822092
- 8
  If for example, the Julian Day for June 22, 2009 at 21:51:35 GMT, then the calculation is simply
  Julian Day : 24550004
  Drop the "24" 550004
  Add the time calculation + 0.910822092
  
  The modified Julian Date is: 55004.910822092

Astronomy