Day of the year and Julian date converter

Ordinal day numbers, YYDDD codes and Julian Day Numbers, in both directions.

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Two "Julian dates" that aren't the same

Ordinal dates (often called Julian dates on shipping labels, pharmaceutical lots and US military documents) are the year followed by the day of the year. They make date arithmetic inside a single year trivial: day 270 minus day 200 is 70 days. ISO 8601 writes them as 2026-270.

Julian Day Numbers count every day since noon on 1 January 4713 BC (proleptic Julian calendar), a starting point chosen by Joseph Scaliger in 1583 so that all historical records would have positive numbers. Astronomers log observations as Julian Days so the gap between two of them is plain subtraction, with no months involved. Wondering why the Moon those astronomers keep timing waxes and wanes at all? ahaboo has a narrated explainer on exactly that.

The Modified Julian Date (MJD = JD − 2,400,000.5) starts at midnight rather than noon and keeps numbers shorter; it's common in satellite and geodesy work.

Day-of-year arithmetic

Day n is the number of days since 1 January plus one. In a leap year every date from 1 March onwards is one day later: 1 March is day 60 in 2026 and day 61 in 2028. For gaps that cross a year boundary, use days between dates.

Questions people ask

What day of the year is it today?
The calculator shows today's ordinal day (1 to 365, or 366 in a leap year), plus how many days are left in the year.
What is a Julian date?
Two different things share the name. In logistics, food packaging and the military, a "Julian date" is an ordinal date: the year plus the day of the year, written YYDDD or YYYYDDD (26270 = day 270 of 2026). In astronomy, the Julian Day Number counts days continuously since 1 January 4713 BC in the proleptic Julian calendar; 1 January 2000 is JDN 2451545.
Is a Julian date the same as the Julian calendar?
No. The Julian calendar is the calendar introduced by Julius Caesar, now 13 days behind the Gregorian calendar. Ordinal "Julian dates" and Julian Day Numbers are just ways of counting days.
How do I convert a YYDDD code for years before 2000?
Two-digit years are ambiguous. This converter reads 00–69 as 2000–2069 and 70–99 as 1970–1999, matching common software. Enter a 7-digit YYYYDDD code to be explicit.

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