Day of year (DOY) is the sequential day number within a calendar year, ranging from 1 (January 1) to 365 or 366 (December 31). This ordinal date system provides a largely unambiguous way to reference dates without month names—useful in scientific data, logistics, agriculture, and programming where numeric day counting simplifies calculations. This tool uses UTC-based calculations to reduce timezone and DST-related offsets; it does not attempt to handle historical calendar reforms.

The system is particularly valuable in fields tracking seasonal patterns. Meteorologists use DOY to compare weather across years (e.g., "temperature on day 100 for the past decade"). Farmers track planting schedules by day number rather than calendar date, as growing seasons follow solar patterns better expressed in sequential days. Astronomers and space agencies use DOY extensively—NASA mission logs often reference "day 185" rather than "July 4."

Leap years add complexity: every year divisible by 4 is a leap year (366 days), except century years unless divisible by 400. So 2000 was a leap year (divisible by 400), but 1900 was not (century year not divisible by 400). This means day 60 is February 29 in leap years, March 1 in regular years, shifting all subsequent day numbers by one until year-end.