Julian Date Calculator
Julian date calculator works out Julian Day and Julian Day Number (noon that day) from What to convert, Calendar date and Time of day (UT). The default figures on this page give julian Day of 2,451,545.000000; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
3 steps
- 2000-01-01 has Julian Day Number 2451545 (the JD at 12:00 UT that day).
- The Julian Day at 12 h UT is 2451545 - 0.5 + 12/24 = 2451545.
- MJD = JD - 2400000.5 = 51544.5.
Its 6 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
4 sources
What the julian date calculator calculates
Converts between a calendar date and the astronomical Julian Day (a continuous day count from -4712-01-01 12:00 UT, so JD 2451545.0 is 2000-01-01 12:00 UT), the Modified Julian Date, and the ordinal "Julian date" stamp used on manufactured goods and in the military (24061 = day 61 of 2024). It runs in both directions and also gives the Julian-calendar equivalent of the date.
How to use this julian date calculator
Enter What to convert, Calendar date, Time of day (UT) and Julian Day. Julian Day updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Julian Day, Julian Day Number (noon that day) and Modified Julian Date.
The method follows IAU / US Naval Observatory, Explanatory Supplement to the Astronomical Almanac, 3rd edition, §15.11 (Julian date), IAU Resolution B1 (1997) defining the epoch J2000.0 as JD 2451545.0 TT, ISO 8601-1:2019, clause 4.1.2.3, ordinal dates (YYYY-DDD) and Howard Hinnant, chrono-Compatible Low-Level Date Algorithms.
The J2000.0 epoch example
With What to convert: date-to-jd, Calendar date: 2000-01-01, Time of day (UT): 12:00, the calculator returns Julian Day: 2451545, Modified Julian Date: 51544.5, Days since J2000.0: 0. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Julian Day, Julian Day Number (noon that day) and Modified Julian Date. The inputs you can vary are What to convert, Calendar date, Time of day (UT), Julian Day, Ordinal date stamp and Ordinal stamp format.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do you calculate Julian date?
Converts between a calendar date and the astronomical Julian Day (a continuous day count from -4712-01-01 12:00 UT, so JD 2451545.0 is 2000-01-01 12:00 UT), the Modified Julian Date. Open “How this is worked out” beside the result to see each step for your own figures.
What are the Julian dates for 2026?
On the figures this page opens with, the julian Day is 2,451,545. No single number fits everyone: it follows What to convert, Calendar date and Time of day (UT), so enter your own above and the answer updates as you type.
How do I convert a date to a Julian date?
It needs What to convert, Calendar date, Time of day (UT), Julian Day, Ordinal date stamp and Ordinal stamp format. Nothing else is required, and the figures stay in your browser.
What is a 4 digit Julian date?
Converts between a calendar date and the astronomical Julian Day (a continuous day count from -4712-01-01 12:00 UT, so JD 2451545.0 is 2000-01-01 12:00 UT), the Modified Julian Date. On this page it is computed from What to convert, Calendar date and Time of day (UT) and shown as Julian Day, Julian Day Number (noon that day) and Modified Julian Date.
What information do I need for the julian date calculator?
You need What to convert, Calendar date, Time of day (UT), Julian Day, Ordinal date stamp, Ordinal stamp format and Read a two-digit year as at or before. The page opens with default figures that give julian Day of 2,451,545.000000, so you can see the scale before entering your own.
What does this julian date calculator show?
It shows Julian Day, Julian Day Number (noon that day), Modified Julian Date, Gregorian date, Time of day (UT), Ordinal date stamp, Day of the year and Weekday, with the working and any warnings for your entries.