I just figured out a cool trick to check when you installed your current Arch Linux install.
All you have to do is check the logs for pacman which can be found at
/var/log/pacman.log. Go to the top of the file and look at the date.
Looks like my current install was installed at
2014-04-16 15:48. Longest
running install yet.
Of course this trick depends on the fact that the logs still exist. If you cleared those logs then you can't really use this trick. Although IMO this is one log you shouldn't delete, considering how valuable its data could be.