Try:

http://www.wikipedia.org/wiki/Unix_epoch (and then http://www.wikipedia.org/wiki/UTC, http://www.wikipedia.org/wiki/Epoch)

And

http://www.google.co.nz/search?hl=en&...y&meta=

Note that the UNIX epoch is based on 00:00:00 UTC on January 1 1970, itself a datetime that doesn't occur at the same instance everywhere in the world.