Treating month and year as fixed spans

A month has 30 days and a year 365 — good enough for calculating.

A month has between 28 and 31 days, a calendar year 365 or 366. Technical calculations therefore use the Julian year of exactly 365.25 days, that is 31,557,600 seconds. A month, by contrast, has no defined length in seconds.

Calendar months and years follow astronomical cycles and are not whole numbers of days. The approximation of 30 days per month is off by up to two days depending on the month, which adds up to several days over a year.

For time spans in seconds use the Julian year when an average is needed, and otherwise calculate with the actual calendar date. A span of 100 years contains 36,524 or 36,525 days depending on where it starts, because the leap-year rule skips the century year.