death certificate

Ok, so I just find this a bit weird, to say the least. My mom passed away on May 17th. I got the phone call at 11:46 p.m telling me she had passed. The hospital form says she passed at 11:40. So the hospital called me quite quickly.

Today I received the Act of Death certificate, and it says that she died at 12:20 a.m May 18th. This has kind of messed me up a bit. I've been thinking that the date I'll remember forever was May17th, but now I have this formal document saying that it was the 18th. 

Is this normal? Does it even matter?