How to use the SUM formula to subtract and calculate time elapsed from mm/dd/yy hh:mm cell

How do I properly use the SUM formula to subtract and calculate time elapsed from mm/dd/yy hh:mm cells?

Example I'm trying to achieve:
Cell A1: 9/27/23 4:15 PM
Cell B1: 9/27/23 4:27 PM
Cell C1: =SUM(B1-A1) to show 00:12
Cell D1: =IF(D1>00:15,NO,YES) to show YES

I've been trying to throw different SUM formulas and custom formatting and this and that, but cannot for the life of me get it to work. I think Office365 web is partially the problem, but perhaps there is something at play.

During the process of trial and error, I've managed to get outputs to show "-4325.4"2 type output or "#######" with no actual numbers. I tried formatting the cell various ways and can't get it to work. Please help.

After getting the correct output going I'm going to throw an IF formula in D1 "=IF(C1>00:15,NO,YES)" to see what tickets are meeting SLA response times.

Update: In the process of adding a sample file, I got =SUM and =SUMPRODUCT formulas to work in two of the cells as samples, but the math is incorrect. Looking forward to learn something about this situation. Thanks a bunch.