Gregorian Calendar

Gregorian calendar - Whoever have coined the term 'Thank God, it's Friday' was a genius! Fridays, if not always, most of the time is great stress-buster. This is a wonderful phenomenon how the dumb looking words and numbers in a calendar becomes a deciding factor to our everyday life. Be it your appointments, birthday, anniversary, or a holiday planner, your most wanted tool to get organize is your friendly calendar hanging from the wall, or sitting pretty at your desk.

It would be a great idea to learn about the evolution of the present day calendar, that is such an indispensable tool in our every day life. Every civilization has contributed to its present shape. Chinese civilization till date follows the lunar calendar, same with the Islamic civilizations, but in a different fashion. The history of present day calendar (or the Christian calendar) can be mainly regarded to be the evolution from Julian calendar and Gregorian calendar. The present day calendar is what Gregorian calendar was formulated.


The Gregorian calendar is the one commonly used calendar worldwide. A modification of the Julian calendar (named after Julius Caesar, who introduced it 45 century B.C), this one was first proposed by the Calabrian doctor Aloysius Lilius, and adopted by Pope Gregory XIII (hence the name) in accordance with the instruction from the Council of Trent (1545-1563) to correct for errors in the older Julian calendar. It was decreed by Pope Gregory XIIIin a papal bull on 24 February 1582.

The Julian Error and the Gregorian Rectification

The Julian calendar introduces an error of 1 day every 128 years. So every 128 years the tropical year shifts one day backwards with respect to the calendar, causing the vernal equinox to slowly drift backwards in the calendar year. Furthermore, the method for calculating the dates of Easter was inaccurate and needed rectification.

The Gregorian calendar was devised to address the above problem. How? The solution to the problem depended on the fact that it was felt that 21 March was the proper day for vernal equinox (because 21 March was the date of vernal equinox during the council of Nicaea in AD 325). The Gregorian calendar was thus configured to make that day vernal equinox. By 1582 vernal equinox had moved (1582-325)/128 days = approximately 10 days backwards. So the 10 days had to be dropped.

Calibration of Gregorian Calendar

One tropical year in this new calendar format decreed by Pope Gregory XIII, is approximated as 365.2425 days, how? Simple, calculate 365 97/400 days. Thus the correction to Julian calendar was made.

Leap Year

The Gregorian calendar has 97 leap years every 400 years. Now let us look how we can determine which one is going to be a leap year.

The Working Formula is Pretty Simple

  • If a given year is divisible by 4 it's a leap year.
  • But, remember, if that given year is also divisible by 100, it is NOT a leap year.
  • Also, if the given year is divisible by 400, it is a leap year indeed.
  • Quite simple, right? Okay, let me illustrate with few examples, 1700, 1800, 1900, 2100 A.D was/is never a leap year, similarly 1600, 2000, 2400 are leap years.


According to Gregorian calendar, each single day comprising of 24 hours is regarded as the basic unit, there by counting the period earth takes a complete revolution round the Sun, it comes to 365 or 366 days, and completes one year. If we take an approximation regarding the average length of a year, it comes around 365.2425 days, or 365 days, 5 hours, 49 minutes, and 12 seconds precisely.

The Gregorian calendar is divided into 12 irregular months to complete a full cycle. January, March, May, July, August, October, and December comprises of 31 full days, while April, June, September, and November comprises 30 days each, and the month of February is used to accommodate the leap day, therefore February can be of 28 or 29 days depending upon whether it's a leap year or not.


So, now every time you look into your calendar, just spare a thought how much calculation has been put into, to make your life easier and simpler, and yes don't forget your wife's birthday.