Convert days to years

One day is 1/365.2425 of a year, because a year has no fixed length. The Gregorian calendar averages 365.2425 days (365, plus a leap day every four years, minus one every century, plus one every four hundred), and that average is what this conversion uses. A particular year is 365 or 366 days, never 365.2425, so this result is an average rather than an exact conversion and should not be used to settle a date. The date calculator does that properly.

Worked through: 365 day x 0.002737907 = 0.999336 year.

Going the other way, one year is 365.2425 days.

For mental arithmetic: Divide by 365 and accept being about a quarter of a percent out.

The approximation is the content here. 365.2425 is the Gregorian mean year, which tracks the tropical year closely enough to drift by about one day in 3,200 years, which is the point of the 400-year correction. For anything where the answer matters to the day, converting through an average year gives the wrong answer and the right-looking one: 1,000 days is 2.7379 average years, but which 1,000 days decides whether that is two years and 270 days or two years and 269.

Precision is worth matching to the input. A measurement given as 30 day carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.