To answer the OP, there is actually a very straight forward solution, which is so obvious, people forget about it!!
When you first turn on your Mac, you need to configure the first user account. If you navigate to the Users folder, and highlight the user account folder and 'get info', it will tell you when that folder was created. This is the date and time your machine would have been switched on (well, to be more precise, a few minutes later).
I also think there is a file somewhere in the main library folder (from root), which details the initial power on but I cannot remember what it is.
Best Regards,
Phil