javascript to sum up time

You can do a simple Sum. It's available under the Calculate tab in the Properties dialog of a text field.

To limit the value to not exceed 60 is more complicated, though. What should happen if it does exceed it?

Do you want to force the user to enter something else into the input fields? Move the part of the value that's beyond 60 to another field? Something else? Either way, that will require using a script.