Epoch time or Unix Epoch time is the number of seconds elapsed since midnight January 1, 1970 (UTC/GMT).
Epoch time is useful for programmers since it's much easier to handle date and time calculations (like adding and subtracting days or hours) when using integers, than actual dates.
Dates are often kept in databases with an epoch representation rather than an actual date format.
Also note that epoch time can be negative so it does support dates earlier than January 1,1970. For example, "-60" represents December 31, 1969 at 23:59:00