For developers

Julian Dates in Code

Copy-paste formulas and functions for both kinds of Julian date — the day-of-year code and the astronomical Julian Date — in the tools you actually use. Each snippet is verified against known reference values.

Excel

Formulas for day-of-year codes and JD — no macros needed.

Python

datetime, strftime %j, and a Meeus JD function (plus astropy).

JavaScript

UTC-safe day-of-year and the JD = ms/86400000 + 2440587.5 identity.

SQL

SQL Server, Oracle, PostgreSQL and MySQL — including Oracle’s J format.

SAS

JULDATE7, DATEJUL and the clean MJD = date + 36934 shortcut.

Java & C#

JulianFields built-ins on the JVM and DayOfYear on .NET.

Two conversions, everywhere

Every page covers the same two tasks in its language:

Day-of-year (ordinal) Julian date — turn a date into YYYYDDD/YYDDD and back.
Astronomical Julian Date — turn a date/time into JD and MJD and back.

Prefer not to write any code? The on-site converter does both instantly in your browser.

A note on accuracy

The astronomical snippets use the standard algorithm from Jean Meeus and agree on the key anchors: the Unix epoch (JD 2440587.5), the MJD epoch (JD 2400000.5) and J2000.0 (JD 2451545.0). Watch out for time-zone and day-boundary details — the pages flag the common traps in each language.