so arithmatic must be done,
If you want to edit the exact value rather than formatting it as a string and then parsing, you are going to have to create a custom control.
The problem is that TimeSpan doesn't do formatting like DateTime.
You are correct...but you can reverse engineer it to place it back into a date time if you need to. If you know the starting DateTime then you can always get back to a date time by just doing the math.
I have a set for the BO property used as a calculated column, but when I set a breakpoint in it, it never hits the breakpoint. Is there something else that I need to do?
I would have to see how your custom property was created, but if you properly implement a custom property with the same attributes as a property created through the BO mapper then a control does not know the difference when bound.