Julian Date Converter

Turn a calendar date and UTC time into Julian date and weekday.

What it does

Astronomers count days without months or years to simplify spans. The Julian date is a continuous count of days and fractions since noon Universal Time on January 1, 4713 BC. Adding 0.5 marks midnight, adding 0.75 marks 6 AM, and so on.

The tool above turns a Gregorian date plus UTC time into Julian date, Modified Julian date, and weekday. The defaults January 1, 2026 at 12:00 UTC match the worked example, so the page opens showing JD 2461042. Type a new date or time to update all three results.

The math follows the standard Fliegel style Gregorian to JD form with month shift, century correction, and time fraction. January and February count as months 13 and 14 of the prior year. The century term B = 2 minus A plus floor(A/4) handles Gregorian leap rules. Time enters as UT hours over 24.

Modified Julian date trims the large leading digits for modern data: MJD = JD minus 2400000.5. The same instant reads smaller and starts at midnight instead of noon. Satellite logs and variable star tables often list MJD for compactness.

This page shows the formula path, one full hand checked example at 2026-01-01 12:00 UTC, a reference table, and practice checks that run through the tool. All date math uses UTC only, so daylight saving never shifts a result.

How to use

1. Pick the calendar date. Dates must exist, so Feb 30 triggers an error.
2. Type the UTC time in 24 hour form. Use 12:00 for a whole JD, 00:00 for .5.
3. Keep years between 1801 and 2099, the range this compact formula covers.
4. Read JD as the main result with up to four decimals.
5. Read MJD for log friendly modern dates.
6. Read weekday to cross check the calendar.

Formula

Y, M, D

Year, month, day, with Jan Feb as months 13, 14 of prior year.

A, B

Century terms: A = floor(Y/100), B = 2 minus A + floor(A/4).

UT

Universal Time in hours, 0 to 24.

After Feb 28, 1900 the extra sign terms in the USNO Almanac form sum to zero, leaving the compact version used here. Leap seconds are ignored. Results use UT1 scale without leap second steps.

Worked example

Reference table

Limits

• Years 1801 to 2099 only in this compact form.
• Gregorian calendar only. Julian calendar dates before Oct 1582 need other tables.
• UTC scale, no leap seconds. High precision timing needs TT or TAI forms.
• Time entry is hours and minutes. Seconds need decimal minutes.

Common mistakes

• Entering local time instead of UTC, shifting JD by the zone offset.
• Forgetting the Jan Feb year shift, which moves results by about a year.
• Reading JD at midnight as whole. Midnight carries .5, noon is whole.
• Mixing JD with Modified JD. They differ by 2400000.5.
• Typing Feb 29 in a common year, which the tool rejects.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• Wikipedia: Julian day
• NASA: Year Dating Conventions

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