"int time ( void )
Returns the current time measured in the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT). "
It has with the time()-function in php, which the board is written in.
edit: or to be more correct really, it has to do with that every time"thingy" has January 1 1970 as a start.
And if your "last visit" isn't set it choose the default-value January 1 1970... or something!

I can't explain very well. Maybe someone else can explain better?