How to format a duration as HH:mm?

There is no need to use formulas for that, you can define your own custom formats.

Just go to Format -> Number -> More formats -> More date and time formats. It will open a window with several date and time formats to choose from. You can define your own as well, using the upper text area:

1. Click on the small arrow on the left, and select the first element you want to add: elapsed hours. Notice there are two different “hours”, one for time and another for durations.
2. Type your separator text :
3. Click on the arrow again and add the second element: Elapsed Minutes.
4. Finally, click on each element you added, to determine the correct format (trailing zeros or not, for example).