The Pregordian Calendar is an adaptation of the Gregorian calendar. The Pregordian date of the year differs from the Gregorian date by at most one day for all dates since the Gregorian Calendar was established. Also the year number is divided into a century and a year CCCCYY, with Gregorian year 1600 corresponding to Pregordian century 1000, year 1 or 100001. The Gregorian year can be used in parallel.

## Leap rules

The main difference with the Gregorian calendar revolves around leap days.

All months of February in the Pregordian calendar have 29 days. On leap years, a leap day is added to December which has 30 days on non-leap years. Most of the time New Year's Eve falls on 30 December, but nothing wrong with celebrating on New Year's day with everyone else when that is the case.

The Gregorian calendar has a mean year of 365.2425 days. The Pregordian calendar has a more accurate mean year of 365.2421875 days. Leap years are calculated as follows:

• If the year is a multiple of 4 other than zero, then it is a leap year.
• If the year is zero then:
• When the century is a multiple of 4 then it is a leap year, except...
• If the century is a multiple of 32 it is not.

The complication is on the century so people only have to think about it every 100 years, otherwise the same effect could have been achieved with a 128-year leap cycle: While the total leap cycle is then 3200 years with 775 leaps in it (compared to Gregorian's 776), the average year is 365 31/128 days long, i.e. 365 days, 5 hours, 48 minutes and 45 seconds. Leap days occur around the same date as in the Gregorian calendar, only two months earlier.

Notable leap and non-leap years
Century Year Last day of December ISO-8601 (Gregorian) date Gregorian leap year Leap day rule
0 0 30 -98400-1-30 (-98400) century % 32 = 0
992 0 30 799-12-31 (800) century % 32 = 0
996 0 31 (leap day) 1199-12-31 (1200) century % 4 = 0
1000 0 31 (leap day) 1599-12-31 1600 century % 4 = 0
1004 0 31 (leap day) 1999-12-31 2000 century % 4 = 0
1004 1 30 2000-12-30 - year % 4 ≠ 0
1004 24 31 (leap day) 2023-12-31 2024 year % 4 = 0
1024 0 30 3999-12-30 4000 century % 32 = 0
1024 1 30 4000-12-29 - year % 4 ≠ 0

The Pregordian year starts one day early on Gregorian non-leap years, so 100401-12-30 (ISO: 2000-12-30) is followed by 100402-01-01 (ISO: 2000-12-31). The two calendars synchronise again on first day of March.

The gap would be wider before ISO-8601 year 800, in which people didn't use the Gregorian calendar anyway, and after ISO-8601 day 4000-12-30 (Prego: 102402-01-01) which doesn’t concern most people anyway. Similar to the Revised Julian calendar, that makes the Pregordian calendar a close match to the Gregorian calendar but with a much improved precision over long eras. However, its leap cycle does not contain a whole number of full weeks.

Moving the leap day from end of February to end of December simplifies date calculations and makes the length of February more even with other months, since it always has 29 days. If you are born on 29 February it may feel like each year is a leap year...

Again, those born on 31 December can celebrate their birthday on New Year's Eve or on New Year's day which falls on 31 December in the Gregorian calendar.