Julian day

The Julian day is a way of numbering days that counts continuously forward from a single fixed starting point far in the past, instead of resetting at the end of a year or a month the way a calendar date does. Because every day gets one running number, astronomers and software systems can subtract two Julian day numbers to find exactly how many days separate two events, without working through leap years, differing month lengths or calendar reforms. The count is used primarily by astronomers for tracking celestial events across long timescales, and by software that needs a simple, reliable way to calculate elapsed days between two dates, such as production and expiration dates, without the complications an ordinary calendar introduces.