I am implementing a calendar/ event booking system where I want to display start date time and a duration or end time.
This is because it’s annoying for the user if you use two date fields, to select the date 
twice, once in a start date time box and in the end date time box, where really the next action after selecting a date and time to start is the end time.
So the ideal would be a field linked to start date time for the date and only displays the time.
I tried to do this with a workflow but it took 10 seconds for the end date time to update with a change on start date time field so that did not work.
I have settled on a duration field but I have no control of it displaying seconds and it is silly that the event is 1:30:10 long 