An epoch reference date is a point on the timeline from which we count time. Moments before that point are counted with a negative number, moments after are counted with a positive number.
Why is 1 January 1970 00:00:00 considered the epoch time?
No, not the epoch, an epoch. There are many epochs in use.
This choice of epoch is arbitrary.
Major computers systems and libraries use any of at least a couple dozen various epochs. One of the most popular epochs is commonly known as Unix Time, using the 1970 UTC moment you mentioned.
While popular, Unix Time’s 1970 may not be the most common. Also in the running for most common would be January 0, 1900 for countless Microsoft Excel & Lotus 1-2-3 spreadsheets, or January 1, 2001 used by Apple’s Cocoa framework in over a billion iOS/macOS machines worldwide in countless apps. Or perhaps January 6, 1980 used by GPS devices?
Different systems use different granularity in counting time.
Even the so-called “Unix Time” varies, with some systems counting whole seconds and some counting milliseconds. Many database such as Postgres use microseconds. Some, such as the modern java.time framework in Java 8 and later, use nanoseconds. Some use still other granularities.
Because there is so much variance in the use of an epoch reference and in the granularities, it is generally best to avoid communicating moments as a count-from-epoch. Between the ambiguity of epoch & granularity, plus the inability of humans to perceive meaningful values (and therefore miss buggy values), use plain text instead of numbers.
The ISO 8601 standard provides an extensive set of practical well-designed formats for expressing date-time values as text. These formats are easy to parse by machine as well as easy to read by humans across cultures.
These include:
2019-01-232019-01-23T12:34:56.123456Z2019-01-23T18:04:56.123456+05:302019-234