According to Waypoint, most of my achievements were earned in 1753. It doesn’t bother me but I thought it was odd.
That happens to achievements you earned offline. If you view them on your Xbox or Xbox.com, they will just say “unlocked” instead of “unlocked mm/dd/yyyy.” Xbox Live only reports dates if you were connected to Xbox Live at the time.
When I earn them offline, I get the default date for the 360 software which is November/something/05 … what did you buy your Xbox off of Ben Franklin or something?