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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.
It tied into there being 365.2422 solar days in the tropical year or 366.2422 sidereal days in the tropical year when I actually found out there were 365.2421822916 solar days in a tropical year or 366.2421822916 sidereal days in a tropical year. 365 days 5 hours 48 minutes 44 seconds and 55 centiseconds per year or 366 sidereal days 5 sidereal hours 48 sidereal minutes 44 sidereal seconds and 55 sidereal centiseconds per year versus 365 days 5 hours 48 minutes 46 seconds and 8 ...