Julian Day Calculator (JD and MJD)

Convert a calendar year, month, day, and time into the Julian Day (JD) and Modified Julian Day (MJD) used in astronomy. Also shows the weekday, day of year, and integer Julian day number.

Input

Enter a calendar year, month, and day to compute the Julian Day (JD) and Modified Julian Day (MJD).

Year

Month

Day

The time in universal time is optional. Any omitted fields are treated as zero.

Hour

Minute

Second

Inputs are treated as a Gregorian calendar date in Coordinated Universal Time (UTC).

Result

Julian Day (JD)

2,451,545

days from noon, universal time

Modified Julian Day (MJD)

51,544.5

Weekday

Saturday

Day of year

1 th day

Julian day number

2,451,545

JD is computed with the standard Gregorian algorithm, and MJD = JD − 2400000.5. The weekday comes from the Julian day number modulo 7, and the day of year counts January 1 as 1.

How it works

The Julian Day (JD) is a continuous count of days starting from noon Universal Time on January 1, 4713 BC in the Julian calendar. Astronomers use it to handle time intervals that span months and years.
Inputs are treated as a Gregorian calendar date in Coordinated Universal Time (UTC). Any omitted time fields are taken as zero.
The integer Julian day number JDN is found as follows. With a = floor((14 − M) / 12), y = Y + 4800 − a, and m = M + 12a − 3, JDN = D + floor((153m + 2) / 5) + 365y + floor(y/4) − floor(y/100) + floor(y/400) − 32045.
To include the time of day, the noon origin is taken into account: JD = JDN + (hour − 12) / 24 + minute / 1440 + second / 86400.
The Modified Julian Day is defined as MJD = JD − 2400000.5, giving a smaller and more convenient value. MJD = 0 corresponds to midnight UTC on November 17, 1858.
The weekday is derived from JDN modulo 7. The day of year is a sequential number with January 1 counted as 1.

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