What is a "month?" (This is probably more a PayPal question)

[sugeneris]

My client just asked this question, and I'm not really sure what the answer is.
He's putting together some subscriber information and was curious exactly what a "month" is when someone buys a subscription. 30 days, 31 days, based on the month the purchase was made in? (We also have 1, 3 & 6-month buttons, for example, so do February buyers get a short one?)

I've found all sorts of gory info at PayPal like what all the form variables mean but can't seem to find anything that directly defines this. Help?