Julian Date Converter
Dates to and from Julian Date, MJD and YYDDD ordinal dates.
Use via API
Convert between calendar dates (UTC), Julian Date (JD), Modified Julian Date (MJD) and ordinal dates (YYDDD / YYYYDDD). Send JSON, get JSON back.
POST https://pithlab.com/api/v1/tools/julian-date-converter
curl https://pithlab.com/api/v1/tools/julian-date-converter \
-H "Authorization: Bearer $PITHLAB_KEY" \
-H "Content-Type: application/json" \
-d '{"from":"calendar","value":"2000-01-01","time":"12:00"}'About the Julian Date Converter
Convert a UTC date and time to a Julian Date, Julian Day Number and Modified Julian Date, or to an ordinal YYDDD date, and convert any of them back.
How to use the Julian Date Converter
- To convert a calendar date, enter the date and, if you need it, a UTC time.
- Read the Julian Date, Modified Julian Date, Julian Day Number and ordinal YYDDD and YYYYDDD forms, each with a copy button.
- To go the other way, choose JD, MJD or ordinal, type the number, and read the calendar date and UTC time.
Examples
Input
2000-01-01 12:00 UTC
Output
2000-01-01 12:00:00 UTC = JD 2451545, MJD 51544.5, ordinal 2000001 (00001)
Input
2026-10-19 18:30 UTC
Output
2026-10-19 18:30:00 UTC = JD 2461333.270833, MJD 61332.770833, ordinal 2026292 (26292)
Input
JD 2460000.5
Output
2023-02-25 00:00:00 UTC = JD 2460000.5, MJD 60000, ordinal 2023056 (23056)
Input
YYDDD 26292
Output
2026-10-19 00:00:00 UTC = JD 2461332.5, MJD 61332, ordinal 2026292 (26292)
Frequently asked questions
What's the difference between a Julian Date and a Julian Day Number?
The Julian Date (JD) is a continuous count of days, with fractions, since noon UTC on 1 January 4713 BC in the Julian calendar. Julian days start at noon, so midnight is always .5. The Julian Day Number (JDN) is the whole number for the day whose noon falls on your calendar date: 2,451,545 for 1 January 2000.
What is the Modified Julian Date?
MJD = JD − 2,400,000.5. It starts at midnight rather than noon (MJD 0 is 17 November 1858) and uses smaller numbers, which is why satellites, astronomy software and many scientific data sets use it.
What is a “Julian date” like 26292?
In manufacturing, food packaging and the military, “Julian date” usually means an ordinal date: the year followed by the day of the year. 26292 (YYDDD) or 2026292 (YYYYDDD) is day 292 of 2026, which is 19 October. This tool reads two-digit years as 2000–2099.
What about dates before 1582?
Dates are entered and shown in the Gregorian calendar, extended backwards before it began on 15 October 1582. Historical dates from before then are usually recorded in the Julian calendar, which runs 10 days behind in 1582 (fewer further back), so convert those first if you need an exact astronomical JD.
Are leap seconds included?
No. Like most software, this treats every day as exactly 86,400 seconds of UTC.