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.
There are 52 weeks in a year but most workers get a two week vacation. So, 8 X 5 X 50 = 2000 hours worked in a year. Many companies also give 10 holidays, which brings it down to 8 x 5 x 48 = 1920. Many companies also give 5 days of sick/personal time per year, which brings it down to 8 x 5 x 47 = 1880.