Day-of-year (ordinal) Julian date
Used on food packaging, in manufacturing and on mainframes: the year plus the day number, 001–365 (366 in leap years).
Calendar date → day of year
Day of year → calendar date
A 5-digit YYDDD code is read as 20YY. The 19YY reading is shown as well.
Astronomical Julian Date
A continuous count of days since noon UTC on 24 November 4714 BC (proleptic Gregorian). The JD ticks over at 12:00 UTC.
Calendar date and UTC time → JD
JD or MJD → calendar date and UTC time
Two meanings of "Julian date"
Day-of-year (ordinal) date. In business, the military and IT, a "Julian date" usually means the year followed by the day of the year. 11 October 2026 is day 284, written 2026-284, 2026284 or 26284. It changes at local midnight, like any calendar date. ISO 8601 calls it the ordinal date.
Astronomical Julian Date (JD). Astronomers count days continuously from an epoch that Joseph Scaliger's Julian Period put at noon on 1 January 4713 BC in the Julian calendar, which is 24 November 4714 BC in the proleptic Gregorian calendar. The whole-number part is the Julian Day Number (JDN), and the fraction is the time since noon UTC. So 2451545.0 is exactly 1 January 2000 at 12:00 UTC (the J2000.0 epoch). The Modified Julian Date (MJD = JD − 2400000.5) starts at midnight and is easier to store.
Neither one is the Julian calendar, Julius Caesar's calendar that the Gregorian calendar replaced from 1582. This page uses the proleptic Gregorian calendar throughout and ignores leap seconds.