So when a school says: The Admission Committee gives full consideration to all completed applications that arrive by the deadline, does it include the deadline?
Yes
If you’re asking if it’s okay to submit on the deadline day… Yes- you will have until midnight. If you’re unable to meet the deadline, I would recommend that you call or email the AO and ask if they’ll accommodate you. Some schools will make an exception and accept an app at a later date … and some won’t. Good luck!
Assume they won’t. In the real world, a deadline is a deadline. Even if they do make an exception, it’s unlikely that the application will receive the same favorable consideration as those submitted on time from students who all had the same amount of time as the late submission to craft their applications.
And don’t wait until 11:59 to hit submit.
Regarding the time when applications are due, I think one needs to be mindful of time zones as well. I could be wrong, but I think Gateway specified a cutoff by time and time zone last year.
I was thinking along the lines of a catastrophic event… like getting hit by a bus but for clarity @skieurope is exactly right. 
What if the teachers are late getting their forms submitted? :((
@Golfgr8 many of the schools we applied to last year had a note about late teacher recommendations, I think it is somewhat common. We were notified just before deadline that several were not received, only to find out the teachers did not realize they had to send out more than one (one for each school) on the Gateway portal…they thought the one would go to all schools he applied to. We corrected it asap, but I think we were just under the wire.
Schools are more understanding of the late teacher recommendations. They know it is somewhat out of your control. Just make sure your application is submitted by the deadline.