"Alert when I need to leave" reminds me 15 minutes too early. How can I turn that off?
I think here is the answer, 



  When you create the event, make sure that you enter the location of the meeting in the event. Like in image, where you can see the greyed out portion with a caption 




 2. 


  Add Location (let it get recognize the event through the map app in
  the background -- you do not have to do anything, this will happen
  automatically based on your internet speed, so just give it some time,
  and once you the location listed, select the desired location from the
  options or the nearby location.)


3.


  Once you do that, if you try to set your event time say
  08:00PM-09:00PM.  Automatically you calendar will recognize this event
  as a remote location and will prompt you differently like this,




This will update you about the meeting at the right time accounting for your traffic and weather conditions.

Good Luck !