The Gregorian Calendar

This section describes The Gregorian Calendar, which is a solar calendar with two types of calendar years: Common year with 365 days and Leap year with 366 days.

The Gregorian calendar is a solar calendar. It follows the tropical year with two types calendar years:

All years are common years with exceptions defined by the following leap year rules:

Now let's see how closely the Gregorian years are synchronized to the tropical years.

Based on leap year rule 1, 4 Gregorian calendar years are about 0.03124 day more than 4 tropical years.

4 Gregorian years: 3 * 365 + 366 = 1461 days 
4 tropical years:  4 * 365.2421896698 = 1460.9687586792 days

Based on leap year rule 2, 100 Gregorian calendar years are about 0.21897 day less than 100 tropical years.

100 Gregorian years: 76 * 365 + 24 * 366 = 36524 days 
100 tropical years:  100 * 365.2421896698 = 36524.21896698 days

Based on leap year rule 3, 400 Gregorian calendar years are about 0.12413 day more than 400 tropical years.

400 Gregorian years: 305 * 365 + 95 * 366 = 146097 days 
400 tropical years:  400 * 365.2421896698 = 146096.87586792 days

As you can see, there will be about one extra day in about 2500 Gregorian calendar years comparing 2500 tropical years.

A Gregorian year is divided into 12 Gregorian months, named as January, February, March, April, May, June, July, August, September, October, November and December. The number of days in each month varies as shown in the following tables for a common year:

Month    1    2    3    4    5    6    7    8    9   10   11   12
Days    31   28   31   30   31   30   31   31   30   31   30   31

For a leap year, the second month has 29 days.

Note that Gregorian month has no relation with the synodic month cycle.

Gregorian calendar also uses the week cycle, which is a 7-day cycle, with days names as Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday. The first day of Gregorian year 1 is a Monday.

Since 400 Gregorian years have 146097 days, which is divisible by 7, the week cycle and the year cycle meets every 400 years.

Table of Contents

 About This Book

Chinese Calendar Background Information

 Astronomical Bases of Calendars

The Gregorian Calendar

 The Chinese Calendar

 Chinese Calendar Algorithm and Program

 Chinese Calendars: Year 1901 to 1910

 Chinese Calendars: Year 1911 to 1920

 Chinese Calendars: Year 1921 to 1930

 Chinese Calendars: Year 1931 to 1940

 Chinese Calendars: Year 1941 to 1950

 Chinese Calendars: Year 1951 to 1960

 Chinese Calendars: Year 1961 to 1970

 Chinese Calendars: Year 1971 to 1980

 Chinese Calendars: Year 1981 to 1990

 Chinese Calendars: Year 1991 to 2000

 Chinese Calendars: Year 2001 to 2010

 Chinese Calendars: Year 2011 to 2020

 Chinese Calendars: Year 2021 to 2030

 Chinese Calendars: Year 2031 to 2040

 Chinese Calendars: Year 2041 to 2050

 Chinese Calendars: Year 2051 to 2060

 Chinese Calendars: Year 2061 to 2070

 Chinese Calendars: Year 2071 to 2080

 Chinese Calendars: Year 2081 to 2090

 Chinese Calendars: Year 2091 to 2100

 References

 PDF Printing Version