iPhone: How do I change the default start time of an event?
Using Siri let’s you nail the start time. The system does still prefer to make a new appointment now if you don’t specify the start time or a start day. 


  Hey Siri, make an appointment for 10 am next Tuesday. 


Also, for when the default calendar is lacking, you can get some very capable apps to extend how you view, automate and combine calendars if you don’t want to just add all your calendars to the native iOS settings and use Siri / calendar.app - the first two I recommend are:


Fantastical 2 - a powerhouse of intelligence and smart scheduling
Calendars 5 - also has better smart scheduling than iOS and has great integration for google calendars and other cloud calendars. 


You don’t need another app, though to just make an appointment precisely when you want it to be slotted in your calendar, just use alternate input mechanisms. These third-party apps had smart scheduling before iOS implemented it, but now Apple ships everything you need so only get these if you need more than one Apple can do out of the box.