How to Calculate in Your Head the Month when You Know the Rest of the Date

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Updated: January 2, 2020

We've all heard of some people who can calculate, quickly (i.e., less than 2 seconds), precisely and in their heads, the Day of the Week for any date. As few as they are in number, virtually none of them can calculate the Month, Day or Year if they know the Day of the Week and two other pieces of any date. This formula will teach you how to calculate the Month when you know the Day, Year and Day of Week.


  1. 1
    Subtract from the day the largest multiple of 7 contained in it (the multiples of 7 are 0, 7, 14, 21, 28, etc.) and hold this number until step 4.
  2. 2
    Subtract from the last two digits of year the largest multiple of 28 contained in it (note: the multiples of 28 are 00, 28, 56 and 84). Divide the result by 4 and ignore any decimal. Add these two numbers together.
  3. 3
    Add to the result in step 2 the value for the century from the Century Table to the right of step 1. (This is the value for the year.)
  4. 4
    Add together the results from steps 1 and 3 and then subtract from the largest multiple of 7 contained in it (if the number is 7 or greater, that is) for a number between 0 and 6. This is the value for the Day and Year.
  5. 5
    Look up the value for the known Day of the Week in the Day of the Week Table. If that value is less than the value for the Day and the Year (step 4) add 7.
  6. 6
    Subtract from the above result the value for the Day and Year from step 3. Look up the result in the Month Table (to the right of step 1) for all months that have that value (but if the year is a leap year, you must subtract 1 from the values shown in the table for January and February, leaving them at 6 and 2, respectively).

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      • This formula is derived from my formula for calculating the Day of the Week in Your Head, which is shown elsewhere in WikiHow (see link below). Please refer to that link for basic Tips.
      • You could hold off on step 1 and just add it in at step 4 instead of going through that "reduce by the largest multiple of 7" twice, but the perceptive among you will figure out why it is suggested that you do it in step 1.
      • Regarding step 6, a year is a leap year if it is evenly divisible by 4 (except that turn of the century years (i.e., '00 years) are not leap years unless they are evenly divisible by 400 (e.g., 1600, 2000, etc.).
      • You may have up to as many as three possible months for some dates since Feb., Mar., Nov. share the same calendar in non-leap years and Jan., April, and July share the same calendar in leap years.



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      Updated: January 2, 2020
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      Categories: Time Calculation
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