That said, I just created a Custom schedule and set it to start and end today, with a specific time range, and it worked perfectly even though the ICAL defaulted to FREQ=WEEKLY. It shouldn't ever repeat, since it is set to a specific date range (today), so perhaps you don't need to change the ICAL...
Also, another thought...why not use the One Time schedule type instead of Custom? Set it to start and end on April 14, and to run All day.