Julian Date Converter (JD, MJD and Day Number)
Convert between calendar dates and Julian Day numbers used in astronomy and satellite work.
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Julian Date Converter (JD, MJD and Day Number) by TheCalcTools
How the Julian Date Converter (JD, MJD and Day Number) works
The Julian Date counts days, and fractions of a day, since noon UTC on 1 January 4713 BC in the Julian calendar. Astronomers use it because it is a single continuous number, without months or years.
Julian days begin at noon, so midnight is .5. The Modified Julian Date subtracts 2,400,000.5 to start days at midnight with smaller numbers.
The formula
Worked example
Using the default values shown in the calculator:
| Convert | Date to Julian Date |
|---|---|
| Date (UTC) | 2026-10-17 |
| UTC time in hours (decimal, e.g. 18.5) | 12 |
| Julian Date | 2.46133e+06 |
- 17 October 2026 at 12:00 UTC is JD 2,461,331.0000 and MJD 61,330.5000.
Result:
| Julian Date (JD) | 2,461,331.0000 |
|---|---|
| Modified Julian Date (MJD) | 61,330.5000 |
| Rata Die (day number) | 739,906 |
| Calendar date (UTC) | Saturday, 17 October 2026 at 12.00 h UTC |
Frequently asked questions
Is a Julian Date the same as the Julian calendar?
No. The Julian Date is a day count; the Julian calendar is the calendar that preceded the Gregorian calendar. Some industries also use “Julian date” to mean the day of the year, which is different again.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.