I've never heard of a half day ticket??? Are you thinking of Epcot??
You pay full price (discounted tickets via websites are still full day though cheaper) regardless of the time you enter. They dont offer a half day ticket on the gates, you will still pay for a whole day even if you enter 2 hours before closing.
Any particular reason for wanting to visit for half a day?? You can get a ticket for ¬21 if you know an AP holder willing to meet you on the day up until mid September.