Dies Domini
Enter either a proleptic Gregorian date or a day number Dies Domini. The other panel updates automatically.
Optional time fields add a fraction of day to the day number Dies Domini.
Dies Domini 0.0 is the ISO date 0000-01-01T00:00:00, using astronomical year numbering and the proleptic Gregorian calendar.
Proleptic Gregorian date
Enter a year, month, and day to calculate its corresponding day number Dies Domini.
Time
Enter either a time of day or a fraction of day. The other field updates automatically.
Day number Dies Domini
Enter a day number Dies Domini to calculate its corresponding proleptic Gregorian date.
Day number and time Dies Domini
A fractional day number appears when a time is entered.
ISO Calendar details
Calendar and numbering rules
The Gregorian bissextile-year rule is extended indefinitely backward and forward: a year is bissextile when divisible by 4, except when divisible by 100 unless it is also divisible by 400. Astronomical year 0 corresponds to 1 BCE in traditional BCE/CE notation.
Day numbers count whole day steps from 0000-01-01 00:00:00. Thus -0001-12-31 is day -1, 0000-01-01 is day 0, and 0000-01-02 is day 1.
When a time or fraction is entered, the “Day number and time” value adds the elapsed fraction of the civil day from midnight. In the Fraction of day field, the decimal point is implied: 5 means .5, and 25 means .25. For example, noon on day 0 is 0.500000000. This resembles Julian Date notation but retains this calculator’s custom epoch and midnight boundary.