DATE-SYSTEM CONVERSION

Gregorian ↔ Julian
Date Laboratory

Convert civil calendar dates to one another and to the continuous Julian Day Number scale.

Gregorian dateJulian civil dateJulian Day Number

SAME INSTANT / CIVIL LABELS

Calendar dates

Proleptic Gregorian—

Proleptic Julian—

Difference for matching Y-M-D—

Weekday—

ORDINAL NOTATION

Day-of-year

Gregorian ordinal—

Julian ordinal—

Gregorian leap year—

Julian leap year—

JULIAN DAY NUMBER

Continuous day count

Gregorian date → JDN—

Julian date → JDN—

This laboratory uses integer JDN values associated with civil dates. The astronomical day changes at noon UT; fractional JD values and time-of-day conversion are not implemented.

JDN INVERSE

Interpret selected JDN

Gregorian civil date—

Julian civil date—

Weekday—

Three names, three concepts

Julian calendar is a civil calendar with a leap day every four years. Julian Day Number (JDN) is an integer count of days used in astronomy and chronology; it is not a calendar system. Ordinal date numbers the day within a year, such as 060 for 29 February in a leap year.

All conversion algorithms here are proleptic: each calendar's rules are extended mathematically across the full supported era. No regional adoption switch is applied. In particular, the 1582 reform's historical skipped dates are not modeled.

BOUNDARY EXAMPLES

Century and reform checks