So this year's tax deadline is July 15th. Does it mean you have to do it by July 14 23:59, or July 15th 23:59. Also what timezone is it in?
From the IRS (emphasis added):
Your return is considered filed on time if the envelope is properly addressed, has enough postage, is postmarked, and is deposited in the mail by the due date. If you file electronically, the date and time in your time zone when your return is transmitted controls whether your return is filed timely. You will later receive an electronic acknowledgement that the IRS has accepted your electronically filed return.
So if you file electronically you have until 11:59 PM in the time zone in which you file the return. If you file by mail it must be postmarked on or before the deadline. I believe most (if not all) post offices will collect mail at midnight on tax day (or will give some grace) to collect and late returns.
That said, don't play with fire. Whether you file electronically or by mail, file early in the day to avoid any penalties.