For each loop with available time slots

For each loop with available time slots

I am very new to Deluge, and this question was unable to be answered by Zoho Creator tech support upon request.

Task at hand:
I have a Form with 4 fields:
- Date Start
- Date End
- Dropdown: Time Start: contains time slots (12:00PM, 12:15PM, etc)
- Dropdown: Time End: contains time slots

When a user submits this form to book an appointment, for example, from 1:00-2:00PM, and this is added to a calendar report, I need it to prevent other users from booking appointments from 1:00-1:45 by removing that option from the Time Start dropdown menu, so long as that appointment exists on the calendar report.

I understand that I need to create a 'for each' loop, making a variable list for both the timeStart and timeEnd fields. How do I go about making a loop that will remove timeStart as an option from the list, then add +1 to remove the next entry, continuing until the value of timeStart = timeEnd, where it will close the loop and not remove timeEnd from the dropdown?

Many thanks.