Well, in computer science - counting time internally using seconds, minutes, hours, days, months would be terribly ineffective, so you just count seconds since certain moment in time and when user requests date displayed, you calculate it (using mktime() or similar function).
On UNIX systems the epoch starts 00:00 January 1, 1970 GMT. Right now it is 109043043 seconds since then and standard long int should suffice to fit such time format till some 2064 or so when we will face another Y2K-alike problem (only harder...)