It shouldn’t be big news that Linux (like Unix) uses Epoch as its internal calendar. Epoch is number of seconds since Jan 1, 1970 00:00 GMT. Wikipedia can tell more.
The problem is just that as a humam it is not really possible to estimate what the epoch values stand for. From sometimes in 2011 to to sometimes in 2014 they start with 13 and have eight more digits. This month they even nicely start with 1345 and 6 more digits. But at least I don’t know of any other rules of thumb to make any more sense out of them.
Of course conversion can be programmed in any language. Or you can go to this online service.
But as a Linux user the following 2 shell commands might be the most handy solution:
$ date -d @1345678901
Thu Aug 23 02:41:41 EEST 2012
$ date -d "Thu Aug 23 02:41:41 EEST 2012" +%s