Calculate Free Slot using Date Difference

Some examples/suggestions:

Conceptually easiest: instead of the field with m/t/w, you should have three records and an integer field to represent the day of the week. This still requires you break down the date ranges for those specific dates.

Can you afford to list each day separately, rather than as blocks? That will also simplify matters. It's much easier to do this as you create the data then to do the calculations (SQL case statements could help if this is not possible).

If times are always fixed on specific borders, I actually use a strategy of start-time and end-time as simple integer references to time slots (a room-reservation application in php/javascript w/SQL database). This showed available times to users by putting up a room, the time slots, and then disabling selection of slots hows numeric value is already within a range for that date/time/room. Every reservation is a single record, cannot span days and is forced to be contiguous by the storage logic (users are warned).

I have more records to handle but each one is conceptually simple: an owner reference, date, starting time block, ending time block. Blocks outside of the valid range do not exist for selection.

You can then use a subquery to create ranges that are NOT covered by the integer ranges, converting the results from integer references to time references for viewing. A data table mapping integers to start-times for fixed-length slots is very helpful for this.

And this is only one possible rearrangement -> others are possible. For example, if you put each day as its own record then you do not need to store the day of the week as it can be obtained from the SQL date functions.