The users would access this from their Android smart phones. This is the rub. The DateTime control is unusable on the smart phone I'm testing on. I can pick a date or click to make hours and minutes increase but decreasing them seems impossible as you can't hold shift and click. (Android 4 device - Note 2).
Every way I've tried to do it without the datetime field runs into gobs of code with deal breakers like;
formula field can't call user function and
formula field can't call formula field.
Any ideas? What online form technology would make this doable?