How does Unix calculate timestamp?

The UNIX timestamp tracks time by using seconds and this count in seconds begins from January 1st 1970. The number of seconds in one year is 24 (hours) X 60 (minutes) X 60 (seconds) which provides you with a total of 86400 which is then used in our formula.

How is timestamp calculated?

The Unix timestamp counts the milliseconds since January 1. 1970 UTC (Coordinated Universal Time). Often computer programs use this value, when calculations with time and date have to be made.