Weird proration calculation explained

[Viktor]

Question:

I have a Charge, with my price conveniently set to $31. If I happen to have a prorated period of 15 days in a 31-day month, I'd expect the charge to be $15, but it's actually $16.61.

How is this calculated?

[Viktor · Accepted solution]

The trick is, it's the start month's number of days that matters. Since the prorated period 03/01/2017 - 03/15/2017 is actually the end of a full period before the next one starts, the full period would be 02/16/2017 - 03/15/2017.

And since the start month is February, and it's number of days is 28, that will be used in the proration formula:

31 / 28 * 15 = 16.6071428

... that is indeed 16.61 if we round it up to two decimals.