If a user trait was setup with a wrong datatype, e.g. as a STRING but its supposed to be DATETIME, then its easy to just delete the trait and recreate it again with the correct datatype.
The problem or confusion comes in, when the trait was previously already used and shows up in the user profile. Deleting the trait will not remove it from the user profile!
The trait is not segmentable, but it will show up in the profile, until it gets recreated again and overwritten with different content. So if the trait got recreated with a new datatype and the user is not updated, it will still show up with the old content.
Only exception is when it was a STRING first and got recreated as a DATETIME - in this case it will actually now interpret the previous content correctly as a date and can also be used in segmentation.