Dates are awkward to subtract: months have different lengths and leap years break every rule of thumb. The Julian Date solves that by numbering every day — and every fraction of a day — in one continuous count. This converter turns a calendar date and UTC time into a Julian Date, Modified Julian Date and several related day counts, and also works backward. It decodes the YYDDD “Julian” codes printed on packaging, logbooks and military forms as well.
How to use the Julian date converter
- Pick what to convert: a calendar date to a Julian Date, a Julian Date (or MJD) to a calendar date, or an ordinal YYDDD code to a calendar date.
- For dates, enter the date and the time in UTC. If you know local time, convert it first, for example with the Zulu time converter.
- For the reverse direction, type the number and say whether it is a JD or an MJD.
- The tape lists the JD, MJD, day of the year, the YYDDD and YYYYDDD forms, the Julian-calendar date, the truncated JD, the Rata Die count and Unix time.
Julian Date formula
The calculator uses the standard algorithm from Jean Meeus’s Astronomical Algorithms. For a Gregorian date with year Y, month M and day D (including the time as a fraction of a day), treat January and February as months 13 and 14 of the previous year, then:
B is the Gregorian correction: it removes the leap days the Gregorian calendar skips in century years not divisible by 400. The related counts are fixed offsets:
Worked example
October 11, 2026, 12:00 UTC.
October is month 10, so Y = 2026, M = 10 and D = 11.5 (noon is half a day). A = 20 and B = 2 − 20 + 5 = −13.
JD = ⌊365.25 × 6742⌋ + ⌊30.6001 × 11⌋ + 11.5 − 13 − 1524.5 = 2,462,515 + 336 + 11.5 − 13 − 1524.5 = 2,461,325.0.
MJD = 61,324.5, it is day 284 of 2026, and the ordinal code is 26284 (YYYYDDD 2026284). The Julian-calendar date is September 28, 2026.
Day counts compared
| System | Starts (day 0 or epoch) | Day begins at | Value for Jan 1, 2000, 12:00 UTC |
|---|---|---|---|
| Julian Date (JD) | Jan 1, 4713 BC (Julian), noon | Noon UTC | 2,451,545.0 |
| Modified JD (MJD) | Nov 17, 1858 | Midnight UTC | 51,544.5 |
| Truncated JD (TJD) | May 24, 1968 | Midnight UTC | 11,544 |
| Rata Die (RD) | Dec 31, 1 BC (Gregorian) | Midnight | 730,120 |
| Unix time | Jan 1, 1970 | Midnight UTC | 946,728,000 seconds |
| Ordinal (YYDDD) | January 1 of each year | Midnight local | 00001 |
Ordinal “Julian” dates
Production lines, pharmacies and the US military often stamp the year and day number instead of a month and day. The day number runs from 001 on January 1 to 365 (or 366 in a leap year) on December 31:
| Date | Day of year (common year) | Day of year (leap year) |
|---|---|---|
| February 1 | 032 | 032 |
| March 1 | 060 | 061 |
| July 4 | 185 | 186 |
| October 1 | 274 | 275 |
| December 31 | 365 | 366 |
Two-digit codes are read with a pivot: 00–49 means 2000–2049 and 50–99 means 1950–1999. Some industries use a single-digit year (6284 for 2026), which this tool does not guess; add the decade yourself.
Calendar notes
The converter treats dates before October 15, 1582 on the proleptic Gregorian calendar — the modern rules extended backward — and shows the Julian-calendar equivalent separately. For weeks rather than days, the week number calculator gives ISO week dates, and the day of the week calculator explains the weekday for any date.
Frequently asked questions
What is a Julian Date?
A Julian Date is a continuous count of days, with fractions for the time of day, starting from noon Universal Time on January 1, 4713 BC in the Julian calendar. Astronomers use it because it turns any date difference into simple subtraction. Noon UTC on January 1, 2000 is JD 2,451,545.0.
Why does the Julian Date start at noon?
The system was designed for astronomers who observed through the night. Starting each Julian day at noon kept a whole night's observations on the same day number. That is why a calendar date at midnight UTC always has a JD ending in .5.
What is the Modified Julian Date?
MJD = JD − 2,400,000.5. Subtracting that constant gives smaller numbers and moves the start of each day back to midnight. MJD 0 is midnight UTC on November 17, 1858, and it is widely used by space agencies and satellite systems.
Is a Julian date the same as the day of the year?
Not strictly, but in manufacturing, food packaging and the military, "Julian date" often means an ordinal date: the year plus the day number, such as 26284 for October 11, 2026. Choose the ordinal option to decode those codes.
Does the Julian Date have anything to do with the Julian calendar?
Only in name and starting point. The day count was proposed by Joseph Scaliger in 1583 and is unrelated to the leap-year rules of Julius Caesar's calendar, though its epoch is expressed as a Julian-calendar date. The converter also shows the Julian-calendar date for historical work.