Help - Event time slot booking Date/Time Problem

Hello, I have been struggling with a problem for a couple of days now and I am hoping someone might be able to help point me in the right direction.
In my app, users can sign up for individual events. Events happen on different days / same day at unique venues and all of these events have their own range of start times available for the individuals to select. Like mini appointments. Essentially every minute within a specified time window which would be ~ less than 5 hours.
I’ve managed to get as far as users being able to select available minutes by matching the duration of each event with a sheet that contains every individual minute up to 5 hours! The problem I’m having is being able to add these available minutes to the actual event start time and as a result displaying real times for the user to select like 17:01,17:02, 17:03 instead of 1,2,3 etc.
The other way I’ve thought about this is to have a sheet that contains the minutes in a day but I can’t seem to get the event start times to match properly with that sheet unless they were also on the same date. Is there any way of removing the date component of a date time column and still be able to do maths with it? Can’t seem to format the cells to only contain Time.

I may be tackling this problem from a weird angle so any help would be greatly appreciated.