The International System of Units (SI) is a measurement system that consists of seven basic units and a large number of derived units that can be expressed in terms of the seven fundamental units.
Any specified time interval used as a standard manner of expressing some duration is referred to as a unit of time.
The SI unit of time is seconds, which is represented by the letter s.
One second was previously expressed as 1 / 86400 of a mean solar day.
The average duration of rotation of the Earth on its axis is called a mean solar day.
This time period can also be expressed in 86400s.
Final answer:
Hence, the S.I. unit of time is seconds.