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.
Over 15 hours long, it is the longest day of the year, aka the Summer Solstice. Science and earthly pleasure collide to celebrate the life-giving power of the earth's closest star: EXACT TIME. Because the earth's rotation varies, the day shifts slightly each year, but it always occurs between June 20 and June 22.