The Calendar on Apple products (iphone, ipad & Macbook pro) has several options to give reminder before event (at event, 5 mts, etc) , or after the same (weekly/monthly/yearly).
It does not have the option to remind during event.
For eg., I am expecting a call from a business associate, between 9-10AM. I do not have an option to remind her at 930 AM or 945 AM, ‘hey, I am waiting for your call’!
It could be simple enhancement ?
Disclaimer: A die hard Apple fan!