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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.
1 So every 4th year we add an extra day (the 29th of February), which makes 365.25 days a year. 2  This is fairly close, but is wrong by about 1 day every 100 years. 3  So every 100 years we don't have a leap year, and that results in 365.24 days per year (1 day less in 100 year = -0.01 days per year).