"Back in my day we played World of Warcraft and Zork on our Commodore 64s! We didn't have any of them fancy schmancy polygons bouncing all over the screen -- we typed out sophisticated sentences such as 'look underneath the bed' to truly immerse ourselves in the text-based interface! Imagination is a helluva drug!" -- Old man voice
It would seem that the reason it says "43 years ago" (instead of any other number) is because the variable for storing "time last logged on" gets set to zero when the person logs out.
Because many computers will use the unix time stamp, setting the last-logged-on to zero corresponds to January 1, 1970 = 43 years ago.
I assume if this bug were to continue until January 2014, it would start showing 44 years ago.
Two flaws with that theory.
1: PC clocks reset to 1/1-1980, not 1970, if there is no battery backup (which have been standard since 1990ish). GetTime functions, does however, return the number of miliseconds since 1/1-70, which is the ony way of telling time in a PC.
2: Back in 2011 and 2012, it also said 43 years..