What is a unixtime, epoch time, or POSIX time stamp?
Unix time is the number of seconds that have elapsed since the Unix epoch, minus leap seconds.
The Unix epoch was set at 00:00:00 UTC on the 1st of January 1970, and each leap second has teh same time as the second before it.
Every day is treated as if it contains exactly 86400 seconds, and because of this Unix time is not a true representation of UTC.
Since solar year comprises precisely of 365 days, 5 hours, 48 minutes, and 45.25 seconds the gregorian calendar
has exactly 365.2425 days.
Human Description Time
† Equivalent to 30.436875 Gregorian days.
‡ Equivalent to 365.2425 Gregorian days.
Year 2038 problem and the Epochalypse
Most legacy systems use 32-bits to represent unixtime which will become a
problem at 03:14:07 on Tuesday, 19 January 2038 when this number representation
will exceed its limited range (0 to 2^32=4294967296).
Before reaching this limit date it expected that most systems will have updated
to a 64-bits representation or been decommissioned.