True that time is a man made concept....Time is a man made concept. The universe doesn't know what time is. There does not have to be a "birth" of the universe. The universe could have always been there. Energy is neither created nor destroyed therefore it is a more than fair assumption that everything in this universe has always been there. "God" did not have to start things. The Big Bang did not have to start things. It could very possibly have always been there. There are more possibilities than your black/white view.
but it isn't true that the universe doesn't know what time is. You don't know the universe could possibly know or not know. Also, we don't know that the universe didn't simply create itself.. creating the laws and time of the universe.
Assuming the universe hasn't always been there is just as valid of an assumption that it has always been there. Thinking that is hasn't always been there is more valid because so far we know of nothing that has Always been.
God, big bang, universe, etc... didn't have to start things--- But there most have been some catalyst - all evidence leads to the idea that there was once a beginning. We are just now finding out about hidden particles in the universe.. so are only recently beginning to understand the formation of the universe....
if big bang is valid then it still doesn't account for the cause of it's inflation.
Then there is probability.
Then there is the 'appearance' of design and fine tuning.
Then there is the fact that we are discussing this as intelligent human beings, living in an intelligent universe. To deny an intelligent catalyst is just like saying inanimate, random matter somehow created intelligence. - this is something that isn't proven and has never happened as far as we know. we know that intelligence creates intelligence for sure.