As a mathematician, I don't think about Big Bang as the "start of time" (or any other dimension), but simply as "fixed point" (of the "physical evolution of all particles in the universe" function).
It's not a fixed point of the equations, though. That is technically incorrect and also not the right intuition. It's the exact opposite. It's a point at which the acceleration of (the density of) all particles diverges, and thus the equations can not be continued past that point.
