I just got a spam email dated 1970/1/1. I laughed, but then thought about it a bit more. I get these from time to time - always spam - and assumed they did this to stand out in your inbox.
But I'm curious how this can happen?
1970/1/1 is the Unix epoch start time, see http://en.wikipedia.org/wiki/Unix_time .
Unix and Linux systems count the time in seconds starting at 1970/1/1, probably, when no date has been set at all in the mail headers, this starting time is set by default by the MTA or showed by default by your mail client.