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.
A regular year has 365 days and a leap year has 366. 366 days in a leap year. I takes the earth 365 and 1/4 days to rotate around the sun. So we add one day every four years to make up for the extra six hours it takes to rotate around the sun each year. Each year normally has 365 days, so we add one for leap years and there are 366 days.