What Is the Modified Julian Date?
The Modified Julian Date (MJD) is a continuous count of days used in astronomy, geodesy, satellite navigation and spaceflight. It is defined as MJD = JD − 2,400,000.5, where JD is the Julian Date. Day 0 of the MJD count began at midnight UTC on November 17, 1858. Whole numbers mark midnight, and the fraction shows the time of day, so MJD 61336.5 is noon UTC on October 23, 2026.
Why Use MJD Instead of the Julian Date?
The full Julian Date has two drawbacks for everyday scientific work. Its days start at noon instead of midnight, a convention inherited from astronomers who observed through the night, and its numbers are large (about 2.46 million today). Subtracting 2,400,000.5 fixes both: MJD days start at midnight like the civil calendar, and the numbers are five digits long for any date between 1858 and 2132. MJD is used by the International Earth Rotation and Reference Systems Service (IERS), in GPS and satellite orbit data, and in many astronomical catalogs.
MJD on January 1 of Each Year
| Date (00:00 UTC) | MJD |
|---|---|
| January 1, 2000 | 51544 |
| January 1, 2020 | 58849 |
| January 1, 2021 | 59215 |
| January 1, 2022 | 59580 |
| January 1, 2023 | 59945 |
| January 1, 2024 | 60310 |
| January 1, 2025 | 60676 |
| January 1, 2026 | 61041 |
| January 1, 2027 | 61406 |
| January 1, 2028 | 61771 |
| January 1, 2029 | 62137 |
| January 1, 2030 | 62502 |
How to Convert MJD
- JD to MJD: subtract 2,400,000.5.
- MJD to JD: add 2,400,000.5.
- Unix time to MJD: divide the Unix timestamp in seconds by 86,400 and add 40,587. The Unix epoch, January 1, 1970, is MJD 40587. See the epoch converter for Unix timestamps.
- Excel or Google Sheets: for a date in cell A1,
=A1+15018gives its MJD, because spreadsheet day 36526 is January 1, 2000 (MJD 51544). Format the result as a number.
MJD vs Other Julian Dates
The term Julian date is used loosely. In manufacturing, the military and food packaging it usually means the ordinal date, a day-of-year number such as 2026-296 or 26296. Astronomers mean the Julian Day Number counted from 4713 BC. MJD is a shifted version of the astronomical count. For today's ordinal date and Julian Day Number, see the Julian date converter or the Julian date calendar for 2026.
