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.
Today, we think a year is about 365 and 1/4 days. We ignore the extra 1/4 and save it up for an extra day every 4 years, which we know as February 29 on leap years. Finally, it is important to realize that time units like day and year are related to reality, something occurring in nature.