Rata Die

Rata Die (RD) is a day-numbering system used in astronomy and calendrical calculations. It represents the number of days since an epoch, specifically since the hypothetical date 0000-12-31 (December 31, year -1) in the proleptic Gregorian calendar. In other words, January 1, year 1 is Rata Die 1. It provides a simple and consistent way to represent any date as a single integer.

The system's main purpose is to facilitate calculating the number of days between two dates. By converting each date to its Rata Die number, the difference between the numbers gives the number of days elapsed between the dates. This avoids the complications of dealing with varying month lengths and leap years directly when performing these calculations.

While it is a convenient system for internal computation, Rata Die is less commonly used for displaying dates to users, who are more accustomed to standard calendar date formats. It serves as an intermediary representation for tasks such as date arithmetic and the conversion between different calendar systems. Its usefulness lies in its linearity and its avoidance of the complexities inherent in conventional calendar structures.

The name "Rata Die" is a pseudo-Latin phrase meaning "fixed day" or "calculated day." It emphasizes the deterministic nature of the numbering system.