You searched this forum, but did you consider searching the Common App Applicant Support Center? Here's what it says:
All application deadlines are as of 11:59pm ET on the stated deadline day. The Common Application records all dates (deadlines, form submission dates, payment dates) in Eastern Time. Schools not located in the eastern time zone may choose to extend their application deadlines to the end of a deadline day in their local time zone, or may accept your documents up until the posted deadline in their local time zone (even though your submission time stamp will be recorded in Eastern Time.) For example, a school located in California may accept your application until 2:59am Eastern Time, which is equivalent to 11:59pm Pacific Time. Since this is on a school-by-school basis and this is not information that may not be available to you, we recommend you submit based on the Eastern Time deadline. Most colleges will accept the submission if it is within a reasonable time period around their deadline.