Missing payments. If you don't see a recent payment you've made, make sure you've allowed enough time for the payment to post to your account: Payments made by electronic check, by credit card, at a T-Mobile store, or over the phone usually post to your account right away, but it can take up to three hours.
Nothing in-game uses Kerbin days/years as far as I can tell. A day is defined as 86400 seconds, and a year as 365 days.
30 days has September, April, June and November. All the rest have 31 Except February alone, Which has 28 days clear And 29 in each leap year. Or you can use the knuckle method: A knuckle is 31 days, and in between each knuckle it isn't.