Modification to Booking System Pro Wordpress plugin

I need a modification to a Wordpress plugin called Booking System Pro: [login to view URL] What you need to do is to make a custom dropdown field on top of the calendar, where customer can choose between 30 minutes and 1 hour intervals. For example: If 30 minutes interval is chosen, available times will be: 9:00 - 9:30, 9:30 - 10:00, 10:00 - 10:30 and so on and if one hour interval is chosen, the available times will be 9:00 - 10:00, 10:00 - 11:00, 11:00 - 12:00 and so on. Booking system Pro has a textarea in backend to input these hours, so you can use this textarea field or you can create your own inside the code, thats your own decision but I need to be able to change these ours. I will send you all the necessary files to make the modification. Ask for further questions.