Time is one of the seven basic physical quantities in the International System of Units. This is a measure, which you can use to arrange past, present and future, and to determine the duration of events and intervals between them. The SI unit of time is the second (denoted as «S»); the units of day, hour, minute are equal to many seconds. Usually there are 365.25 days in a year. All of the important units of time are interrelated.