Many people know when their half birthday is: 6 months after their birthday. But have you ever looked at the date and saw your birthday’s day in a different month and thought, “I wonder how old I am right now?” 

My birthday is May 10, so I’ll use that as an example. I know my half birthday is November 10, because it is 6 months after May and 6 months before May. If there are an equal number of months on either side of the month, that month must be halfway. But another way to think about November as being 6 months after May out of a possible 12 months. 6/12 can be simplified to 1/2, hence the half birthday. 

What about December 10? If 6 months passed to get November, then 7 months passed to get December. That makes December my 7/12 birthday. Quite an ugly number, but it allows for the conversation starter: “I can’t believe no one wished me a happy 7/12ths birthday!”

In January, 8 months out of 12 have passed, so that’s my 8/12 birthday. 8/12 is 2/3 if you simplify. 

In February, 9 months out of 12 have passed. 9/12 is 3/4! If you see me on February 10, you’d better wish me a happy 3/4 birthday. 

March 10 is therefore my 10/12ths birthday or 5/6ths.

April is 11/12ths (almost there!).

And finally, May 10 is my 12/12ths birthday, or my 1st birthday since May 10 last year. 

Next time you see a random month with the day of your birthday, calculate which birthday that is!