How many days in a year

One year is 365.2425 days here, the average length of a year in the Gregorian calendar. Every actual year is 365 or 366 days. The average comes from the leap-year rule: a year divisible by 4 is a leap year, except a year divisible by 100, unless it is also divisible by 400. That gives 97 leap years in every 400, and 365 plus 97/400 is exactly 365.2425. So the factor is exact as a description of the calendar and an average as a description of any one year.

Worked through: 1 year x 365.2425 = 365.243 day.

Going the other way, one day is 0.002737907 years.

For mental arithmetic: 365 for most years, 366 for leap years, 365.25 as a quick average.

The rule has three parts because each fixes the error the previous one left. Julius Caesar's calendar added a leap day every four years, for an average of 365.25 days. That is about 11 minutes longer than the solar year it was meant to track, which is roughly 365.2422 days. Eleven minutes a year is a day every 128 years, and by the 16th century the calendar had drifted ten days from the seasons. The Gregorian reform of 1582 dropped those ten days and added the century exception, which is why 1900 was not a leap year, 2000 was, and 2100 will not be. Britain and its colonies did not switch until 1752, by which time the gap was eleven days. The Gregorian year is still about 27 seconds longer than the solar one, which adds up to a day in roughly 3,200 years.

Precision is worth matching to the input. A measurement given as 0.1 year carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.