Method Deck / dates

Weekday of a date

Computes the day of the week for any proleptic Gregorian calendar date.

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Inputs

• date (string, required) : ISO 8601 date, YYYY-MM-DD

Outputs

• weekday: Name of the day of the week

Steps

1. Parse the date as a proleptic Gregorian civil date (year, month, day).
2. Compute its day-number using the Hinnant days_from_civil algorithm (equivalent to a Julian Day Number offset), which counts whole days since 1970-01-01.
3. Take the day-number modulo 7. 1970-01-01 has day-number 0 and was a Thursday, so the remainder indexes into the cycle starting at Thursday.
4. Map the remainder to a weekday name using that calibrated cycle.

Constants

• anchor_date = 1970-01-01 date (source)
• anchor_weekday = Thursday weekday name (source)

Sources

• chrono-Compatible Low-Level Date Algorithms (Howard Hinnant, 2026-09-30T19:41:35Z) - days_from_civil day-number kernel; 1970-01-01 is day-number 0 and is independently known to have been a Thursday

Worked example and test vectors

Published card

• Address: b05a15968f2f5d61c84c549313ab0900
• Made at: 2026-09-30T22:39:30.722Z
• Method sha256: d2245aad8a43e5a1d0059ae77babc58f5d457d3b37ff4c5187d66c203eee6679