When I create a service, it would be pretty important to be able to give it a duration of 0min as well. Some of our services are additional services that do not take more time, but bring additional income. When you have 1min to choose from, it confuses the entire calendar. We've ended up rather putting in that straight 15min duration because the calendar stays better on its schedule that way. We now remove it ourselves 15min if we want from certain services after a customer has booked, but it's pretty unintelligent to have to observe this.