Is there an Excel formula for calculating a length of time and converting it to decimals?

Math hurts my brain, and I am getting tired of counting on my fingers and adding fractions. Help a jelly out?

What I need is a time-specific formula for a horizontal layout something like this:

Cell A1: I’d enter the ‘start time’ (for example, 1:15 pm)

Cell B1: I’d enter the ‘end time’ (say, 7:45 pm)

Cell C1: Here, I want it to automatically calculate the time between A1 and B1, in standard quarter hour decimals (in this case, that would be 6.5 hours).

Ideally, I’d like a formula that can do the job even when the start time is in the PM and the end time is in the AM (for example 11:45 pm to 3:30 am, which is 3.75 hours). I always round the start and end times to the nearest 15 minutes when I enter them, so rounding isn’t an issue in the formula.

PS: I did search for the formula, but couldn’t find what I’m looking for. I might just be using the wrong terms, though.