Julian Date Converter (JD & MJD)
A Julian Date (JD) is a continuous count of days and fractions since the epoch of January 1, 4713 BC (Julian calendar) at 12:00 UTC — today's JD is around 2,461,000. The Modified Julian Date (MJD) shifts the epoch to 1858-11-17 and subtracts 2,400,000.5 for a smaller, midnight-based number.
Julian dates are used wherever exact day arithmetic across centuries matters: astronomy, satellites, GPS-era systems and aerospace software. This converter returns JD, MJD and the IDL Julian variant (MJD in IDL's convention) for any input date.
Julian Date Converter (JD & MJD) Quick Reference
| Format | Pattern / Description | Example 1 | Example 2 |
|---|---|---|---|
| Julian Date (JD) | Days since 4713 BC | 2461266.10417 | 2461041.50000 |
| Modified Julian Date | JD - 2400000.5 | 61265.60417 | 61041.00000 |
| IDL Julian Date | Scientific | JD2461266.10417 | JD2461041.50000 |
* Examples are rendered in UTC for the fixed sample instants above.
When to Use Julian Date (JD & MJD)
Astronomy & observatories
Observation logs, ephemerides and data archives are keyed by JD. Convert your observation time to JD before matching catalog entries.
Aerospace & satellite systems
Mission timelines and orbit calculations use JD/MJD as a continuous time axis, avoiding calendar arithmetic entirely.
Exact date difference
JD makes "days between any two dates" a single subtraction, including across leap years and calendar reforms.
Frequently Asked Questions
What is Julian Date?
JD is the continuous day count since noon (12:00 UTC) on January 1, 4713 BC, a convention introduced by Joseph Scaliger in 1583 for aligning historical calendars.
What is the difference between JD and MJD?
MJD = JD − 2,400,000.5, starting at midnight instead of noon (epoch 1858-11-17). MJD numbers are ~5 digits today (e.g. 61,090) versus ~7 digits for JD (2,461,090).
What is the Julian date of today?
For 2026-08-13 the JD is ≈ 2,461,265 and MJD ≈ 61,265. Use this converter for the exact fractional value at any time of day.
Is Julian date the same as the Julian calendar?
No. The Julian calendar is a calendar system (leap year every 4 years). Julian Date is a day-counting system that happens to be named after its inventor's father — the two are often confused.
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