Calendar Date Lookup Tool

A single calendar date carries several independent derived values. The ordinal day of year numbers dates from 1 to 365, or 366 in a leap year, and is obtained by differencing against 1 January. The ISO 8601 week number follows a specific rule: weeks begin on Monday, and week 1 is the week containing the first Thursday of January, equivalently the week containing 4 January. A year has 53 such weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year, which is why late December dates sometimes belong to week 1 of the following year. Leap years are those divisible by 4, except centuries not divisible by 400, giving a mean year of 365.2425 days against a tropical year of about 365.2422. Two continuous day counts are also shown. The Julian Day Number counts days from 1 January 4713 BC in the proleptic Julian calendar and is used in astronomy because it removes calendar reform, era boundaries and unequal months from interval arithmetic, so differences between dates are simple subtraction. The Unix timestamp counts seconds from 1 January 1970 UTC. Both carry caveats: Julian Day Numbers computed with a proleptic Gregorian formula will not match dates as recorded in Julian calendar sources before 1582, and Unix time ignores leap seconds and overflows on 19 January 2038 where it is still stored in a signed 32 bit integer.

Calculating a weekday by hand was a recognised problem long before computers. Christian Zeller, a German mathematician and theologian, published the congruence that bears his name in the early 1880s, reducing the weekday of any Gregorian or Julian date to a short modular expression. John Conway devised the Doomsday algorithm in 1973 around anchor days that recur within each year, trading generality for the ability to compute a weekday mentally in seconds. Both remain in use, though software normally converts to a serial day number and takes the remainder modulo 7. The serial counts themselves are older and newer respectively. Joseph Justus Scaliger proposed the Julian Period in De emendatione temporum in 1583, choosing an epoch in 4713 BC where three chronological cycles coincide, and named it after his father Julius rather than after the Julian calendar. Unix time dates from the early 1970s at Bell Laboratories, where the epoch settled at 1 January 1970 after an initial scheme that counted sixtieths of a second and would have exhausted its range far too quickly. ISO 8601 unified the written formats, appearing in its first edition in 1988 and superseding the earlier ISO 2014 numeric date standard of 1976.