Question: why epoch time is used?
Answer: The first edition Unix Programmer's Manual dated November 3, 1971 defines the Unix time as "the time since 00:00:00, Jan. 1, 1971, measured in sixtieths of a second". Because of [the] limited range, the epoch was redefined more than once, before the rate was changed to 1 Hz and the epoch was set to its present value.