# How to Mentally Calculate the Difference between Two Dates

## Method 1

(This is an untested, brainstormed idea. Josh (talk) 21:02, 5 January 2013 (CST))

1. Calculate the number of full years between the dates
2. Multiply that by 365
3. Calculate the number of full leap years between the dates
4. Add the number of full leap years to the total, because a leap year has 366 days -- one extra day per full leap year
5. Then calculate the number of days before and the number of days after the total of the full years. See below.

### Number of Full Years

Each year has 365 days, except for leap years which have 366. Start by figuring out how many full years are between the two dates. Multiply that number by 365.

Then you need an extra day for every full leap year, so calculate the number of full leap years and add that to the total number of days.

### Number of Extra Days

Then you need the number of extra days on each side. By memorizing the next two tables, you should be able to do the calculation more quickly. prev_1_1 is the number of days from the first of the month to the previous January 1st (1/1). next_1_1 Is the number of days from the first of the month to the next January 1st (1/1).

prev_months and next_months can be used to calculate the number of months, in case you want to give the answer in a format like: "16 years, 4 months, and 3 days" instead of "5966 days".

#### Regular Years

Day prev_1_1 next_1_1 prev_months next_months
January 1 0 365 0 12
February 1 31 334 1 11
March 1 59 row 2, cell 3 2 10
April 1 90 row 2, cell 3 3 9
May 1 120 row 2, cell 3 4 8
June 1 151 row 2, cell 3 5 7
July 1 181 row 2, cell 3 6 6
August 1 212 row 2, cell 3 7 5
September 1 243 row 2, cell 3 8 4
October 1 273 row 2, cell 3 9 3
November 1 304 row 2, cell 3 10 2
December 1 334 row 2, cell 3 11 1

#### Leap Years

This is for leap years:

Day prev_1_1 next_1_1
January 1 0 366
February 1 row 2, cell 2 row 2, cell 3
March 1 row 2, cell 2 row 2, cell 3
April 1 row 2, cell 2 row 2, cell 3
May 1 row 2, cell 2 row 2, cell 3
June 1 row 2, cell 2 row 2, cell 3
July 1 row 2, cell 2 row 2, cell 3
August 1 row 2, cell 2 row 2, cell 3
September 1 row 2, cell 2 row 2, cell 3
October 1 row 2, cell 2 row 2, cell 3
November 1 row 2, cell 2 row 2, cell 3
December 1 row 2, cell 2 row 2, cell 3

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