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.
Each knuckle month has 31 days, while each valley month has only 30 days (excepting of course February). February has a smaller leftover amount of 28 or 29 days because pre-45 B.C. it was the last month of the Roman calendar year which started every March.