Or does it have to be around April 16 at 11:59pm.. Pacific time..
Before midnight April 17
"Turn in" makes no sense.
It has to be postmarked midnight on the 17th.
Yes. April 16 is emancipation day in DC so all government offices are closed so the deadline is the 17th. Most post office will stay open until midnight to postmark letters.
It can be postmarked up to 11:59 PM on the 17th