Julian Day Converter
Turn a calendar date into a Julian day number, Julian date or Modified Julian date, and back again.
Results
Julian day number
2461303
Julian date at midnight UT
2461302.5
Modified Julian date
61302.0
Days since J2000.0
9757.5
Day of the week
6
Calendar date for that Julian day
1 Jan 2000
Hour of that Julian day, in UT
12.00
What this tool does
Astronomers gave up on months a long time ago. A Julian day is a single running count of days that started in 4713 BC, which makes the interval between two observations a subtraction instead of a calendar problem. Two details trip people up: the count rolls over at noon rather than midnight, so a Julian date ending in .5 is a midnight; and the Modified Julian date simply chops off the first 2 400 000.5 to give a number that fits comfortably in a spreadsheet. Type a date to get its Julian day, or type a Julian day to get the date back.
Formula
MJD = JD − 2 400 000.5 , days since J2000.0 = JD − 2 451 545.0
Variables
| Symbol | Meaning | Unit |
|---|---|---|
reference_date | Reference date | — |
julian_in | A Julian day to turn back into a date | — |
JDN | Julian day number | — |
JD | Julian date at midnight UT | — |
MJD | Modified Julian date | — |
J2 | Days since J2000.0 | — |
WD | Day of the week | — |
FD | Calendar date for that Julian day | — |
FH | Hour of that Julian day, in UT | — |
Worked example
- Reference date2026-09-19
- A Julian day to turn back into a date2,451,545
- Julian day number2461303
- Julian date at midnight UT2461302.5
- Modified Julian date61302.0
- Days since J2000.09757.5
- Day of the week6
- Calendar date for that Julian day1 Jan 2000
- Hour of that Julian day, in UT12.00
Limitations
- Dates are handled in the proleptic Gregorian calendar, which is the one computers use. For dates before the Gregorian reform of 1582 this differs from what was written at the time.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.