How to make Timing slots and feel it into dropdown list

I want to create the slots of timing for example intime of doctor is 09:00 am and out time is 05:00 am then i want the drop down down list to be filled with slots like 09:00-09:15

                                                                                                                09:15-09:30

so on till the 04:45-05:00 . Time is stored in varchar format in database . Then How to make the slots?