In the "Trigger after update" property of the time field 'Start Time' I can set the 'End Time' to be the same as 'Start Time' with 'End Time' := 'Start Time'
How would I set the 'End Time' to the 'Start Time' + 30 minutes?
'End Time' :=number( 'Start Time')+(30*60*1000)
Works! Thanks Leo.