Date-Math Fail

A few cycles back, I realized something while counting which blew my mind with just how terrible I am at date math.

The average cycle is 28 days, and ovulation most frequently occurs two weeks before the next period in an average cycle. That’s what I usually use, and good thing too, because I was trying to cut a corner and translate that into days-from-last-period and decided it meant Day 14 in an average cycle.

But if the first day of the next period is thought of as day 29, then two weeks before that is day 15. That’s of course also two weeks after day 1, but having a background in math, sometimes I instinctively count from 0 even when it’s not appropriate.

I was staring at a calendar trying to figure out if I’d forgotten how to add, when it clicked that I was either using Day 0, which doesn’t exist in this model, or incorrectly using Day 28.

You can imagine what happens when I start trying to count cycles against real months. Hence the creation of the Pregnancy Calendar Generator.

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