It should be a decimal indeed.
In a MF you can use the function minutesBetween
More info on this is in the documentation:
you can create a microflow that recieves a customer. And the you can create a variable with this value:
This gives you the total of seconds between those dates. The you can transform it into (HH:MM:SS). I saw this idea in https://docs.mendix.com/refguide/between-date-function-calls
Even this link can help you in the next steps: https://docs.mendix.com/refguide/parse-and-format-date-function-calls
I hope this can help you.
There is no straight forward way to transform the difference into (HH:MM:SS). You will need to follow these steps in a microflow: