This is equivalent to :
What is epoch/unix time? Unix time is a system for describing a point in time. It is the number of seconds that have elapsed since the Unix epoch, that is the time 00:00:00 UTC on 1 January 1970, minus leap seconds. Leap seconds are ignored, with a leap second having the same Unix time as the second before it, and every day is treated as if it contains exactly 86400 seconds. Due to this treatment, Unix time is not a true representation of UTC.
Two layers of encoding make up Unix time. The first layer encodes a point in time as a scalar real number which represents the number of seconds that have passed since 00:00:00 UTC Thursday, 1 January 1970. The second layer encodes that number as a sequence of bits or decimal digits.
Epoch/Unix time vs Human-readable time
All these timestamps are in seconds, if you would like to see milliseconds add three zeros (000).