year
Timeexact by definition
The year used here is the Julian year, exactly 365.25 days of 86,400 seconds, which is exactly 31,557,600 seconds. That value is exact by definition and is the year astronomy uses, notably in the definition of the light-year. It is close to but not identical with the tropical year or the calendar year.
Watch out: Several different years are in circulation. The Julian year is exactly 365.25 days; the tropical year, governing the seasons, is about 365.2422 days; the Gregorian calendar's mean year is 365.2425 days; and the sidereal year, one orbit relative to the stars, is about 365.256 days. They differ in the fifth significant figure, which is negligible for a utility bill and not for an ephemeris.
| 1 yr | 31,557,600 s |
About the year
There is no single length of a year, which is why the unit has to be stated. The tropical year is the one the seasons follow, the interval between successive March equinoxes, about 365.2422 days. The sidereal year is one full orbit measured against the fixed stars, about 365.256 days, and it is longer because the Earth's axis precesses slowly, shifting the equinox backwards along the orbit. Astronomy needs a year that never changes, so it defines the Julian year as exactly 365.25 days and uses it for the light-year and for epochs like J2000.0.
The calendar is a separate problem: fitting a non-integer number of days into whole-day years. Julius Caesar's reform used 365.25 days with one leap day every four years, which overshoots the tropical year by about 11 minutes annually. Over sixteen centuries that accumulated to ten days, and the equinox had drifted far enough from its assigned date to disturb the calculation of Easter. The Gregorian reform of 1582 dropped ten days and changed the leap rule so that century years are leap only when divisible by 400, giving a mean year of 365.2425 days. That is within about 27 seconds of the tropical year, good for roughly three thousand years before another correction is needed.