My understanding is that time is motion. Every single way that we measure time involves motion. Whether its the movement of the hands on a watch or the vibrations of an atom in an atomic clock. It is all motion. Without -any- motion, there is no time. So, at the time just before the Big Bang, the universe was the size of a mathematical point—a singularity. No motion occurred inside this point (that would be impossible). Since no motion occurred at that point, time did not exist yet. Time begins once things start to move.
As Alamo_Girl has put it on previous occasions, ‘Without Time, events do not occur; without space, things do not exist.’