Method Deck / dates

Julian Day Number of a civil date

Converts a proleptic Gregorian civil date to its Julian Day Number (JDN).

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Inputs

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

Outputs

jdn: Julian Day Number, an integer day count with no calendar gaps

Steps

Parse the date as a proleptic Gregorian civil date.
Convert it to a day-number using Hinnant's days_from_civil algorithm, which is arithmetically equivalent to the Fliegel and Van Flandern (1968) integer JDN formula.
Add 2440588 (the JDN of 1970-01-01, the algorithm's epoch) to obtain the Julian Day Number.

Constants

epoch_jdn_1970_01_01 = 2440588 JDN (source)

Sources

chrono-Compatible Low-Level Date Algorithms (Howard Hinnant, 2026-09-30T19:41:35Z) - days_from_civil; verified against the well-known fact that 2000-01-01 is JDN 2451545
A Machine Algorithm for Processing Calendar Dates (Fliegel, H. F.; Van Flandern, T. C., Communications of the ACM 11(10):657 (1968), 2026-09-30T19:41:35Z) - the original integer civil-date-to-JDN algorithm this method is arithmetically equivalent to

Worked example and test vectors

Name	Inputs	Answer
worked example	`{"date":"2000-01-01"}`	2451545
test vector: Unix epoch	`{"date":"1970-01-01"}`	2440588
test vector: recent date	`{"date":"2024-01-01"}`	2460311

Published card

Address: c7f1588e7d1e5143d8f697fdcbfcc10e
Made at: 2026-09-30T22:39:30.722Z
Method sha256: c300e9aaa1ec27c3b141cb0cf719516b287f32e985d8eba4d33c059f67934b83

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