How you determine your age?
I was born in 1987 and this is 2011, so I am 24 years old right now.
How most computer systems determine your age?
Time passed since 1st January 1970 till now (2011)
Time passed since 1st January 1970 till when you were born (1987)
(well, again) 24
The date 1st January 1970 is the Epoch date for the computer, signifying the start of time for it. That’s why timestamps in most programming languages are the seconds/milliseconds elapsed since the epoch.