Julian Date Calculator

The Julian Day rolls over at noon UT, so 12:00 gives a whole number.
Astronomical Julian Day, e.g. 2451545.0 for the J2000.0 epoch.
Five digits (YYDDD) or seven (YYYYDDD).
A two-digit year is ambiguous; it is read as the latest year no later than this that ends in those digits.
Julian Day
2,451,545.000000
Julian Date Calculator
Julian Day Number (noon that day)
2,451,545
Modified Julian Date
51,544.500000
Gregorian date
2000-01-01
Time of day (UT)
12:00:00
Ordinal date stamp
00001
Day of the year
1
Weekday
Saturday
Same day in the Julian calendar
1999-12-19
Gregorian ahead of Julian by
13 days
Days since J2000.0
0.000000
Summary
2000-01-01 12:00:00 UT = JD 2451545 = MJD 51544.5; ordinal stamp 00001.
Worked out from IAU / US Naval Observatory, Explanatory Supplement to the Astronomical Almanac, 3rd edition, ยง15.11 (Julian date) and 3 other sources.

How this is worked out

3 steps
  1. 2000-01-01 has Julian Day Number 2451545 (the JD at 12:00 UT that day).
  2. The Julian Day at 12 h UT is 2451545 - 0.5 + 12/24 = 2451545.
  3. MJD = JD - 2400000.5 = 51544.5.

Results are general information, not financial, tax, legal, medical or engineering advice.