get Epoch Second
Returns the number of seconds since the UNIX epoch (1970-01-01T00:00:00Z) if this value is positive, or until the UNIX epoch if this value is negative.
For example, this value will be -1 for the instant 1969-12-31T23:59:59Z, and 1 for the instant 1970-01-01T00:00:01Z.