Exactly the problem with the theory. It's like Plato's world of forms and every scientific and religious explination for the beginning to the world since. Nobody knows, so they transfer the problem.
The problem is not that nobody knows - the problem is nobody can properly grasp it!
Your day to day experience of time is not general. That is to say when things start to get extreme the rules change - Very high speeds, very high gravitational forces "slow down time"
This is not theory it's been proven time and time again - my favorite example is that we measure far more of a type of particle called a muon at the surface of the Earth than we should. They travel so fast that time slows for them and they manage to reach us before decaying.
Anyway -time is inextricably linked to space - which is why we call it space-time; the big bang was not a giant star that exploded into an existing viod but the very start of space and time itself.
The problem is that our very language does not equip us to deal with such concepts - in early school we are told every sentence requires a verb - a "doing word" but that itself has a concept of time in it, verbs, words like "before", "after" all of this has no meaning when you are considering the "start of time".
So basically the answer - and I know it's a very unsatisfactory one is that the very question itself has no meaning - a bit like asking what you were doing before you were conceived
An example I like to use for relativistic time effects is that GPS satellites have atomic clocks specially calibrated (before launch) to run a little slow in order to remain in sync with clocks on Earth so that they can continuously relay accurate positioning data in spite of the time they "gain" from orbital velocity.
If interested you can download an article on Relativity in the Global Positioning System here