Wikipedia: Calendar math

Calendar math
From Wikipedia, the free encyclopedia.

Do you know what day of the week you were born on? Most people don't know and their parents cannot remember. But here is a math formula that you can do to find out what day of the week you were born on.

Corresponding months

"Corresponding months" means that a month will start on the same day as another month. September and December correspond, meaning that September 1 is on the same day that December 1 is on. All months will not start on the same day. The only way a month can start on the same day every month is if the number of days in the month is divisible by 7. Here's how each month corresponds:

January corresponds to October in common years, but corresponds to April and July in leap years.
February corresponds to March and November in common years, but corresponds to August in leap years.
April corresponds to July, and to January in leap years.
May, June, and August ordinarily doesn't correspond to any month.
September corresponds to December in common and leap years.

Feburary corresponds to March because February has 28 days, a number divisible by 7. November corresponds to February because January and October correspond, and both have 31 days.

Corresponding years

According to the Gregorian calendar, there are 7 possible ways that a year can start on, and 7 possible ways for a leap year to end the year (so there are just about 14 different ways for a year to turn out).

"Corresponding years" works in the same way as corresponding months, however on a bigger perspective. For example, 2003 was a common year starting on Wednesday, meaning that 2003 corresponds to the 1997 calendar year. 2004, on the other hand, is a leap year starting on Thursday, meaning that the year starts off corresponding to 1998 and ends corresponding to 1999.

What day of the week were you born on?

Here is the formula for calcuating what a day of the week a date was on:

Let's use April 24, 1982 as an example.

Enter the last two digits of the year: 82.
Multiply the number by .25 or divide it by 4: 82x.25=20.5. Drop the decimal if there is one.
Find the number corresponding to the month you were born (look below for months table): April=0
Add all numbers from steps 1-3 to the date you were born: 82+20+0+24=126.
Divide the sum you got in step 4 by 7: 126/7=18
Find the remainder in the days table: 0=Saturday.

Months Table

Month 1800-99 1900-99 2000-99 January 3* 1* 0* February 6** 4** 3** March 6 4 3 April 2 0 6 May 4 2 1 June 0 5 4 July 2 0 6 August 5 3 2 September 1 6 5 October 3 1 0 November 6 4 3 December 1 6 5

*2,0,6 in leap years **5,3,2 in leap years

Days Table

Sunday 1 Monday 2 Tuesday 3 Wednesday 4 Thursday 5 Friday 6 Saturday 0

See also: ISO 8601, Dominical letter