JR
I assume it's because the page was loaded before a post's datestamp and the username data was entered in the database (server going sluggishly?), making the homepage automatically reset to the UNIX epoch which is midnight 1970. A similar effect can be achieved if you remove the CMOS battery from many computers, or leave them unplugged for a long amount of time, where the system clock will reset to 1970.
If you really were that bored, here's the Wikipedia article.
If you really were that bored, here's the Wikipedia article.