One More Question About Spacetime: Why did Time only expand forwards after the Big Bang?

Space and time are one thing.
After the Big Bang spacetime expanded in all directions.
We know that space observably expanded in all directions.
But how come time only expanded forwards?
(How do I know time only expanded forwards? Because scientists say there was no time before the big bang and if Time had expanded backwards there would be time before the Big Bang just as there is time after it.)