Reply To: Half day and full day reservations

No that can’t be done at all. I think the best solution would be to add a two selects in html to the form that have the same names like the hour fields.
The first select with name date-from-hour:
9am (value:9)
1pm (value:13)
Second select with name date-to-hour:
1pm (value:13)
5pm (value: 17)

Then they can select only from your options and the availability check works completly.