Epoch Time

"Unix time (also known as POSIX time or Epoch time) is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, not counting leap seconds. It is used widely in Unix-like and many other operating systems and file formats. Because it does not handle leap seconds, it is neither a linear representation of time nor a true representation of UTC. Unix time may be checked on most Unix systems by typing date +%s on the command line." 

Example: 1440999387 (ISO 8601:2015-08-31T05:36:27Z)

Unix time - Wikipedia - https://en.wikipedia.org/wiki/Unix_time

See also:
 * https://en.wikipedia.org/wiki/Epoch_%28reference_date%29
 * http://stackoverflow.com/questions/1090869/why-is-1-1-1970-the-epoch-time