Unix Time is the total number of seconds since the Unix Epoch began on 01/01/1970 at 00:00:00 (UTC +0).
Computers use Unix Time as a standard measure of time, as Unix Time is based on the UTC (Coordinated Universal Time) world time standard, which means that Unix Time will not change between different timezones.
As Unix Time is stored as a Signed 32-bit integer, it cannot contain a value greater than 2147483647, doing so would result in a 32-bit integer overflow which would result in (-)2147483648, which would cause computers to interpret the date/time as Fri, 13 Dec 1901 20:45:52, The counter "runs out" of usable bits, "increments" the sign bit instead, and reports a maximally negative number (continuing to count up, toward zero). Resulting erroneous calculations on such systems are likely to cause problems for users and other relying parties..