I looked at this again today and discovered a difference:
On Date/Time field, Validate fires on exit even if no change was made to the data.
On other type fields, Validate fires on exit when data was changed.
READER 8.0 and up:
On Date/Time field and other type fields, Validate fires on exit when data was changed.
I read in the Livecycle reference manual that the Validate event is supposed to fire on exit only when there is a change to the data. So does the older Acrobat Reader versions have a bug with regard to this?