How to Enter/Edit only Hour & Minute (e.g 08:30 or 17:45) portion of a DateTime field in Zoho Creator Form?

How to Enter/Edit only Hour & Minute (e.g 08:30 or 17:45) portion of a DateTime field in Zoho Creator Form?

1. In my TimeSheet application I have two time fields say From Time and To Time AND I want enter/edit ONLY the Hour & Minute portion of a Date/Time filed as 08:30 or 1730 OR 12.15. Whatever way I entered the time should be formatted/visible as 08:30 hidding the date and second portion.
 
2. My objective is to calculate the time ( Hours & Mininutes) between these two datetime fields.
 
3. FYI, I have been able to create a CALCULATED Field showing the time as 08:30 by using the SubString function BUT cann't use it in normal field and enter as above.
 
4. It would be better if ZOHO team can add the MS Access Input Mask feature not only for this Time Data Entry but also for other types susch Phone Number Product Number etc.
 
Regards,
MMI