If you do not want your staff to have the ability to book odd hours, only full days or half days, then you need to restrict the holiday bookings. To do this go to your Company Settings
and choose the Holidays tab
You will then see the title Manage holiday bookings. If you leave Flexible bookings checked then your staff will be able to book odd hours and minutes.
If you do not want them to have this ability then check Restricted bookings and they will only be able to book full or half days. If you are part way through your holiday year and you want your staff to be able to use their odd hours, then you can set a date when you would like restricted bookings to come into effect. Do not set a date if you want them to be restricted immediately. Admins are able to override full or half day bookings to book smaller amounts of time on the holiday booking form.