[SllX]

Friday Midnight is the same as Midnight Saturday Morning.

I mean there is no standards committee for this, but that's what common convention tells us in the Anglophone part of the world.

[FabHK]

Is "midnight 1st January 2017" unambiguous? Would it be in a legal document?

I've often enough seen "1st January 2017, 11:59 pm" or, even worse, "1st January 2017, 12:01 am" for deadlines etc.

[SllX]

Would you rely on common conventions in a legal document rather than legally defined terms?