This might be related to the UNIX epoch.
Time in a UNIX system is measured at a 1 Hz rate, time 0 is defined as 1970-01-01.
http://en.wikipedia.org/wiki/Unix_epochMy guess is that a realtime-clock perpahs does not save the first to digits of the year (19, 20), but something like. Since the time pre 1970 is invalid in a UNIX system, one might assume that if the real time clock is pre "70", the year is 20xx.