First of all one must define time. As physically understood time is a ‘dimension’ in what we could call a ‘space’ with four dimension (three spatial dimensions and one temporal).
If we have a graph where we plot ‘events’ as points time is just one co-ordinate in such graph.
Regarding ‘time being flexible’, I am sure you mean time is not an absolute co-ordinate but the perception of time depends on the observer, as the theory of (special) relativity tells us.
Although time is relative there are some ‘fixed rules as well’. For example the ‘speed of light limit’ for any “information” prevents time paradoxes (things that do not carry information can go faster than light, like a shadow for example, or virtual particles, this will not violate special relativity).
What was time at the start of the Big Bang? Who can answer that one?
Well SOME scientists, like Krauss and Hawking wrote recently two books where they claim to have the answer. Now their answer is quite debatable at a scientific level (since not all scientists agree with their conclusions) and plain wrong philosophically (although in their books they claim philosophy is useless and dead they indulge themselves in sophomoric and puerile philosophizing).
The MAIN POINT is that the universe does NOT need to begin with the Big Bang to need a creator. Actually the universe could be itself infinite in a temporal sense as well (like in the ’ Big Bounce Model’… which is now thought to be wrong however) and have no temporal beginning, yet the same arguments that many (like Aquinas) provide for the need of the creator would still stand.
So creation is definitively NOT ‘temporal’ in the sense that God started the Big Bang and that was it. That is more a ‘deist’ view, not a theist or Christian view.