CalculationTime

Julian Day Calendar

A printable UTC converter for Julian Date, Julian Day Number, Modified Julian Day, Unix day count and month-by-month Julian day tables beside the CalculationTime clockface.

Time Duration Calculator

Julian Day Builder

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CalculationTime Julian Day Calendar

JD 2,451,545.00000

Sat, 01 Jan 2000, 12:00 UTC uses the astronomical Julian Day count from noon UTC.
JDN
2,451,545
MJD
51,544.50000
UTC Date
2000-01-01
Rata Die
730,120
Julian Date2,451,545.00000
Julian Day Number2,451,545
Modified Julian Day51,544.50000
Unix day count10,957.50000

January 2000

Saturday 01 January 2000 to Monday 31 January 2000
Day 1
DateWeekdayJDN at noonMJD at midnight
2000-01-01Sat2,451,54551,544.0
2000-01-02Sun2,451,54651,545.0
2000-01-03Mon2,451,54751,546.0
2000-01-04Tue2,451,54851,547.0
2000-01-05Wed2,451,54951,548.0
2000-01-06Thu2,451,55051,549.0
2000-01-07Fri2,451,55151,550.0
2000-01-08Sat2,451,55251,551.0
2000-01-09Sun2,451,55351,552.0
2000-01-10Mon2,451,55451,553.0
2000-01-11Tue2,451,55551,554.0
2000-01-12Wed2,451,55651,555.0
2000-01-13Thu2,451,55751,556.0
2000-01-14Fri2,451,55851,557.0
2000-01-15Sat2,451,55951,558.0
2000-01-16Sun2,451,56051,559.0
2000-01-17Mon2,451,56151,560.0
2000-01-18Tue2,451,56251,561.0
2000-01-19Wed2,451,56351,562.0
2000-01-20Thu2,451,56451,563.0
2000-01-21Fri2,451,56551,564.0
2000-01-22Sat2,451,56651,565.0
2000-01-23Sun2,451,56751,566.0
2000-01-24Mon2,451,56851,567.0
2000-01-25Tue2,451,56951,568.0
2000-01-26Wed2,451,57051,569.0
2000-01-27Thu2,451,57151,570.0
2000-01-28Fri2,451,57251,571.0
2000-01-29Sat2,451,57351,572.0
2000-01-30Sun2,451,57451,573.0
2000-01-31Mon2,451,57551,574.0

Formula Path

JD = Unix milliseconds / 86,400,000 + 2,440,587.5. JDN = floor(JD + 0.5). MJD = JD - 2,400,000.5.

Noon Boundary

The Julian Day changes at noon UTC. The calendar table labels each civil date with the JDN containing its noon instant.

Notes

Observation log, archival citation, dataset index, telescope notes or conversion cross-checks.

Method

Julian Days count continuous days from an astronomical epoch

The tool treats the input as a UTC instant, converts browser milliseconds to Julian Date using the Unix epoch offset, then derives Julian Day Number and Modified Julian Day from that same instant.

Noon start

Astronomical Julian Days begin at noon UTC, so Julian Day Number changes at noon, not at local midnight.

Month table

The printable table lists each UTC civil date with its noon JDN and midnight MJD for archival cross-checking.

Limits

This is a proleptic Gregorian UTC converter from 1583 onward. It does not model historical local calendar adoption dates or time-scale corrections.

Source Notes

Julian Date, JDN and MJD are astronomy and data-index conventions

The United States Naval Observatory describes Julian Date as a continuous day count used by astronomers, while Modified Julian Day is an offset variant often used for compact date indexing. CalculationTime uses browser UTC Date values only for the mechanical conversion.

ReferencesUSNO Julian Date FAQIAU measuring time overviewECMAScript Date objects