Visa valid until 19-04-2016 meaning

Hi all,
I am currently in China on a S2 type that is valid until the 19th of April. My flight back home leaves the 20th on 00.40. So I figured that I would have passed immigration by the 19th so that the 19th would be my 30th day in PRC. But the S2 type I got does not say 30 days valid, but it says that my S2 type is valid until the 19th. At the moment I am very confused with the meaning of until. Does this mean that I can legally be in PRC on the 19th of April, or does it mean that I have to leave PRC before the 19th. And will my plan of going through immigration on the 19th work if my flight is just passed midnight? It is a large flight to Europe, so boarding will start somewhere around 00.00 I guess.

I really hope someone can help me with these questions, because I really rather not overstay my S2 type.