Decimal Year Calculator

About this calculator

A decimal year expresses a calendar date as a single fractional number, calculated as year + (day-of-year − 1) / days-in-year — so June 30 of a 365-day year lands around 2026.496, roughly halfway through the year. Leap years use 366 in the denominator, which shifts the decimal value slightly compared to non-leap years for the same calendar date.

This calculator converts any calendar date into its decimal-year equivalent, correctly accounting for leap years. Decimal years are the standard time format in astronomy (for star catalog epochs), geophysics (for GPS and tectonic plate motion data), and scientific datasets where a single sortable, continuously increasing number is more convenient than separate year/month/day fields.