Simplify date and time

I have created an input screen that asks for a "start time' and "end time" for projects. Then, the program is to automatically calculate how much time is spent on the project. I want the times simplified so that I enter only hours and minutes, NOT seconds. It would be even better if the inputted time was automatically rounded up to the nearest 1/4 hour.

How do you do that? Also, is there a simple way to have the total time calculated?