Unix Epoch Time is the number of seconds since 1970-01-01 00:00:00 UTC (Coordinated Universal Time zone). This arbitrary moment in time is defined as “the beginning”, and this timezone used as the reference.
The more you work with times, dates, time zones and daylight saving time, the more you will come to appreciate constant points of reference like this. Below is a tool for working with and understanding dates and times and epoch numbers.