That's usually because the date header was missing completely, and some
system along the way assigned a header of Jan 1 1970 -0000, then
corrected for timezone shift.

Possible. But that doesn't explain the ones dated 2001.

My theory: A lot of spam is sent by computers that have been taken
over by spam sending viruses. Computer owners who aren't bright
enough to set the time right are also not bright enough to install
anti-virus software or take other precautions to prevent their
computers from being turned into spam slaves, ergo lots of bad
times on spam :-).

P.S. http://home.att.net/~Tom.Horsley/ntptime.html is the web
page for my NTP client to keep your windows system time set if
anyone is interested.