These are [Unix timestamps](http://en.wikipedia.org/wiki/Unix_time). They represent the number of seconds that have passed since the Unix Epoch, which occurred at 12:00 am UTC on January 1, 1970.

You can convert a human-readable date to / from a Unix timestamp using [Wolfram Alpha](http://www.wolframalpha.com). Example queries:

> "convert January 15, 2005 14:56 PST to unix timestamp"  
> "convert unix timestamp 1105829760"