Mendix has choosen to only provide type DateTime as an option. There is no option for just the time. For your requirement:
- for Start time to your entity add an attribute of type datetime
- for Finish time to your entity add an attribute of type datetime
- for TimeTaken to your entity add an attribute of type decimal and calculate the time taken using the function millisecondsbetween, see https://docs.mendix.com/refguide/between-date-function-calls#2-millisecondsbetween
you can display your date time attributes on the front end using custom format to display only time.