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Within these tables, January 1 is always the first day of the year. The Gregorian calendar did not exist before October 15, 1582. Gregorian dates before that are proleptic, that is, using the Gregorian rules to reckon backward from October 15, 1582.
The authors design individual calendrical calculation algorithms for converting each of these calendars to and from a common format, the Rata Die system of days numbered from January 1 of the (fictional) Gregorian year 1. Combining these methods allows the conversion between any two of the calendars.
This makes R.D. more suitable for work on calendar dates, whereas JD is more suitable for work on time per se. The systems also differ trivially by having different epochs: R.D. is 1 at midnight (00:00) local time on January 1, AD 1 in the proleptic Gregorian calendar , JD is 0 at noon (12:00) Universal Time on January 1, 4713 BC in the ...
The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a leap day being added to February in the leap years. The months and length of months in the Gregorian calendar are the same as for the Julian calendar.
Leap years have two letters, so for January and February calculate the day of the week for January 1 and for March to December calculate the day of the week for October 1. Leap years are all years that divide exactly by four, with the following exceptions: Gregorian calendar – all years divisible by 100, except those that divide exactly by 400.
The golden number of any Julian or Gregorian calendar year can be calculated by dividing the year by 19, taking the remainder, and adding 1. (In mathematics this can be expressed as (year number modulo 19) + 1.) For example, 2025 divided by 19 gives 106, remainder 11. Adding 1 to the remainder gives a golden number of 12.
The Lilian day number is a count of days of the Gregorian calendar and not defined relative to the Julian Date. It is an integer applied to a whole day; day 1 was October 15, 1582, which was the day the Gregorian calendar went into effect. The original paper defining it makes no mention of the time zone, and no mention of time-of-day. [25]
A calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days. The Gregorian calendar year, which is in use as civil calendar in most of the world, begins on January 1 and ends on December 31 . [ 1 ]