Calendar Feed - Half day holiday adjustment

I have my company work pattern as 9.00am to 5.30pm with half an hour for lunch. Each employee works 8 hours per day Monday to Friday. Half a days holiday should mean employees work 4 hours leaving at 1pm if wanting an afternoon off and starting at 1.30 if wanting the morning off. The calendar feed is currently showing a 12pm finish when processing half a days holiday which is wrong and i cant seem to change it.