Year
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.
Day
A day, symbol "d", defined as 86 400 seconds, is not an SI unit, but is accepted for use with SI. The Second is the base unit of time in SI units.