Number Cards handling time

Hi, we are working on a add-on for the ERPNext HR Module in which we will add some features that we are missing in the original version. You can follow our progress here:

For test cases I was trying to set up a dashboard in which we would like to show some information about working hours etc.

We will want to track working hours in the time format hh:mm:ss as this is best readable by humans.

When creating a a number card, I notices, that I can not make a number card with “average” based on a field of the type “time”.

Is there any reason, why this is not possible? Am I missing some logical understanding for this, or has it just not been implemented, as it has not been necessary in any use case yet?

Looking forward to hearing from you all.