I would agree with the caveat that the Big Bang technically began slightly after T=0, so there's still an extremely short frame of time where we can ask questions about what caused the Big Bang. There are a variety of theoretical models with different explanations.
From our perspective, if we went back in time before the Big Bang, it would seem like there was infinite time due to how spacetime itself shrinks the further back you go. When we actually model time this early, it approaches T=0 through a curve without ever actually touching it.
Time isn't actually infinite during that period, importantly. It would just seem that way to someone moving backwards through time, due to time itself shrinking.
Just as eternity is in our hearts, for every second we can count we can count both forwards and backwards forever and ever.