Dies Domini

Calendars. The proleptic Gregorian calendar uses astronomical year numbering, including year 0, which corresponds to 1 BC; year -1 corresponds to 2 BC. Its bissextile-year rule is extended indefinitely backward and forward: years divisible by 4 are bissextile, except years divisible by 100 unless they are also divisible by 400. The proleptic Julian calendar is displayed in BC/AD notation and has no year zero: 1 BC is followed by AD 1. Its bissextile rule is every fourth year when expressed in astronomical year numbering. Both calendar panels represent the same absolute civil day and therefore update each other.

Dies Domini. Day number 0 begins at Gregorian 0000-01-01 00:00:00, equivalent to Julian 1 BC January 3 00:00:00. Whole day numbers change at midnight: Gregorian -0001-12-31 is day -1, 0000-01-01 is day 0, and 0000-01-02 is day 1.

Time and fractions. A time adds the elapsed fraction of the civil day since midnight to the Dies Domini day number. The Fraction of day field has an implied decimal point, so 5 means .5 and 25 means .25. For example, noon on day 0 is 0.500000000. The fraction is stored to nine decimal places. This notation resembles a Julian Date, but it is not an astronomical Julian Date: it retains the Dies Domini epoch and a midnight day boundary.

ISO details. The ISO date is based on the proleptic Gregorian calendar. ISO day numbers run from Monday = 1 through Sunday = 7, and ISO week 1 is the week containing the year's first Thursday. When a time is present, the ISO date displays whole seconds only. Fractional seconds entered directly are truncated; when the time comes from the nine-digit Fraction of day field, a value within half of that field's smallest step of an exact second is treated as that exact second so equivalent time and fraction entries remain consistent.