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.
With the exception of a leap year, each month has exactly the same number of days each year. Ranging from 28 to 31 days each month, the Gregorian Calendar used in the United States measures the number of days and weeks in a year for a total of 365 days -- or 366 days in a leap year.