Wikipedia Unix Time: "The Unix epoch is the time 00:00:00 UTC on 1 January 1970."
PST = UTC - 8, so 00:00 UTC = 16:00 or 4PM PST.
I've seen a few of these kinds of emails, and what I think is happening is a "drive-by delivery". The spammer, instead of trying to get some proxy to deliver his dastardly message, does it himself but lazily doesn't add all the normal headers (date, subject, to, from), just inputs the envelope information to the SMTP listener, and then puts in a basically zero or 2-byte (blank) message. Lacking a date header, your email program translates the datestamp "0" to your time-zone. Which in your case happens to be a significant date. You probably didn't realize that it was an auspicious moment in more than one "epoch"!
I think these are either dictionary attacks, a spammer-software failure, or some other harvester at work. I have at times received spam messages via this method, and the receiving system always adds its received header, but the rest of the message is missing any significant headers. Whatever email system you're using must not be showing the full raw message, because the receiving system has to put its received header in. Unless its a really brain-dead non-compliant system!