How can I change the default 1 hour duration of a new appointment in the iOS calendar? My calendar appointments automatically default to an hour for the end of appointment. I am running on iOS 11 on my iPhone. 

How do I change the default time to just 1/2 hour?
I don’t see anything in the settings app for changing this for the calendars.  Only the default reminder times have a preference. 

I use Siri for this and it works great for me 


Hey Siri, make an appointment at 10 A.M. for 15 minutes.
add a recurring appointment for 30 minutes starting at 9 A.M. on the 28th of every month


change the title to review finances