Definition
Two layers of encoding make up Unix time. These can usefully be separated. The first layer encodes a point in time as a scalar real number, and the second encodes that number as a sequence of bits or decimal digits. As is standard with UTC, this article labels days using the Gregorian calendar, and counts times within each day in hours, minutes, and seconds. Some of the examples also show International Atomic Time (TAI), another time scheme, which uses the same seconds and is displayed in the same format as UTC, but in which every day is exactly 86400 seconds long, gradually losing synchronization with the Earth's rotation at a rate of roughly one second per year.
Source: Wikipedia
UTC basis
The present form of UTC, with leap seconds, is defined only from 1 January 1972 onwards. Prior to that, since 1 January 1961 there was an older form of UTC in which not only were there occasional time steps, which were by non-integer numbers of seconds, but also the UTC second was slightly longer than the SI second, and periodically changed to continuously approximate the Earth's rotation. Prior to 1961 there was no UTC, and prior to 1958 there was no widespread atomic timekeeping; in these eras, some approximation of GMT (based directly on the Earth's rotation) was used instead of an atomic timescale.
Source: Wikipedia
Unix Timestamp: 1730937928