You can only do that with Schwinns built in '76 and later having a 4-digit number stamped on the headbadge. That number is in the form (dddY) where ddd represents the ordinal day and Y represents the last digit of the year. Schwinn first started recording the day of assembly in 1974 specifically for the purpose of being able to handle recalls, but for the first two years Schwinn only provided that information to its dealers.

The serial number date code site you mentioned is here. Note that the serial number recorded dates only indicate when the *frames* were made. The actual bikes were normally built weeks, months or in some cases even years later. Schwinn built frames in batches and more limited production frames (e.g. tandems, fillet-brazed models) tended to sit in inventory longer than high-volume frames before being painted and built into bikes.

With a frame built that late in '64 it is very likely your bike wasn't built until some time in '65, as a '65 model of course. Was your Twinn originally a single, 2-speed or Deluxe Twinn 5-speed? If it was originally a 2-speed or Deluxe Twinn 5-speed then it is a '65 since those models weren't available until then: Schwinn catalogs, 1961 - 1970 (267 of 765)

I've got several Schwinns that were built 1, 2 and 3 model years after their frames were made and have documented other examples with up to 6 years difference between frame and bike builds.