daynumber

Daynumber is a term used in calendrical and computational contexts to denote the sequential count of days relative to a reference point. It can refer to a position within a year, or to a continuous count across many years.

Two common senses of daynumber exist. The first is the day-of-year, or ordinal date: the day number within a calendar year, starting at 1 for January 1. In non-leap years the range is 1 to 365; in leap years it extends to 366. This sense is used for scheduling, versioning, and in date representations such as one that writes year and day-of-year together, for example 2024-128.

The second sense is a continuous day count such as the Julian day number or epoch day count used in astronomy and computing. The Julian Day Number (JDN) is a count of days since a fixed epoch, traditionally noon UTC on January 1, 4713 BCE. Modern practices also employ related counts like the Modified Julian Date. These measurements enable straightforward interval calculations and calendar conversions.

In computing, daynumber often appears as a variable name for a field storing an integer day count. Many programming languages and libraries offer functions to compute either sense: extracting the day-of-year from a date, converting to days since an epoch, or converting between calendar systems. Examples occur across Python, Java, and database query languages.

Notes and caveats include the impact of time zones on time-of-day values but not on day counts when calculations use calendar boundaries. Historical calendar reforms can complicate cross-era conversions, and leap-year rules vary by calendar. In modern software, the proleptic Gregorian calendar is commonly used for deriving daynumbers.