I have a Yahoo Email from December 8.
December 8 2027.
How could the Emailer do this?
obviously its for top billing. Any other reasons?
Was the email sent from a Yahoo account or into your Yahoo account, or both?
If sent from a non-Yahoo account into your Yahoo account, the sending server has a date problem.
Then again, Yahoo is based in California where just, … well, … things are different.
I have to ask; what was in the email? 
Simple: Fill that date into the email’s ‘Date:’ header field.
What, you thought there was some sort of verification involved? Nah. Email server software doesn’t even verify the ‘From:’ field, which is why it’s always a lie in spam email.
If you wanted to read headers (at least possible in GMail, I don’t know about Yahoo email) you might get a better idea of when it was sent, unless it never left a local machine (and thus outside computers never had a chance to leave a more reasonable date stamp) or all of the machines it passed through left similarly bogus date stamps.
Lots of spammers spoof the date field so that their message stays at the top of your inbox.
What’s special about the year 2027? I had a computer/file system/date glitch which left a bunch of my files on the computer dated 2027. Is it a special year in computer terms?