The only time API field in the Activity object is the ActivityDateTime. If I want to capture the Start Time ina formula. I don't see a solution. the following formula is pulling the end time. So it Start =2:30 and end time is 3:30, it displays 3:30:
IF( OR( VALUE( MID( TEXT( ActivityDateTime - (1/24) ), 12, 2 ) ) = 0, VALUE( MID( TEXT( ActivityDateTime - (1/24) ), 12, 2 ) ) = 12 ), "12", TEXT( VALUE( MID( TEXT( ActivityDateTime - (1/24) ), 12, 2 ) ) - IF( VALUE( MID( TEXT( ActivityDateTime - (1/24)), 12, 2 ) ) < 12, 0, 12 ) ) ) & ":" & MID( TEXT( ActivityDateTime - (1/24) ), 15, 2 ) & " " &IF(VALUE( MID( TEXT( ActivityDateTime - (1/24) ), 12, 2 ) ) < 12, "AM", "PM" )