The POSIX time, or Unix time, is the number of seconds elapsed from the midnight of January 1st 1970 in UTC coordinates. That zero moment, known as the epoch, is simply the start of the decade in which the Unix operating system (which first used this time representation) was invented.
Unix time is used widely in Unix-like and many other operating systems and file formats. Many Unix systems store epoch dates as a signed 32-bit integer, which might cause problems on January 19, 2038 (known as the Year 2038 problem or Y2038).
The unix time stamp is a way to track time as a running total of seconds. This count starts at the Unix Epoch on January 1st, 1970 at UTC. Therefore, the unix time stamp is merely the number of seconds between a particular date and the Unix Epoch. The time technically does not change no matter where you are located on the globe. This is very useful to computer systems for tracking and sorting dated information in dynamic and distributed applications both online and client side.