You can’t do that with the java datetime formatter, which offers no duration options.
You could display hh:mm using $H:$M but it will go wrong when duration exceeds 24 hours.
But I don’t think any of that works on your String type Item, because the state is not held as a datetime for the formatter to work with…
You’ll want a transformation to do this properly, probably a javascript that calculates epoch seconds and then does maths division to get the time values you want.