Build date?

The first '65 Mustang supposedly rolled off the line on March 9th, 1964. If you are referring to the regular '65 Alternator-equipped cars, then that would have been around the beginning of August '64.

Remember that the date on your door dataplate is the "scheduled build date" and could have been off as much as days or weeks before or after. Usually, if the car has unique options or trim codes out of the norm, then the actual build date was usually skew from the scheduled date.