If you are not sure, you should ask the school specifically. Like others said, many schools clarify their deadline by giving an exact time, such as "by Noon April 15" or "by 11:59:59PM April 15" or "by the end of business day April 15". To me, if no time is given, I would read the deadline as 5PM on April 15 at the school's local time. Typically, in academia, a deadline is given such that the date written/spoken is the last day in which one can take an action. You might remember the application deadlines to work the same way. Typically, you see grant deadlines, conference deadlines, etc. written where if the deadline is May 1, that means May 1 is the last day in which you can take that action.
