Historical Note about the Date Type

The first official Gregorian calendar day occurred on the date of its inception on 15 October 1582 (Gregorian). This reform was later adopted by most western cultures at different times. We can still identify particular days before 15 October 1582 (Gregorian) using dates in the Gregorian calendar, simply by projecting the Gregorian dating system back beyond the time of its implementation. A calendar obtained by extension earlier in time than its invention or implementation is called the proleptic version of the calendar, and we therefore obtain the Proleptic Gregorian Calendar.

The Proleptic Gregorian Calendar has a year 0, and there are no years Before Christ (BC). The year before 1 Anno Domini (AD) is 0, and the year before that is -1.

The Julian day number of the Date primitive type is the number of elapsed days since the start of the Julian period, as defined by Joseph Scaliger. The start of the Julian period, established by Scaliger, is 1 January 4713 Before Common Era Proleptic Julian (BCE). The Julian period is the universal cycle (or period) used in chronology, especially for astronomical calculations involving large time intervals.