system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people say that time began at the big bang? Is there no 1 trillion years ago, for example?
gpt
People say that because, as far as we can tell, that's what happened. "Time" isn't a thing by itself, it's intimately connected with space (that's why physicists talk about "spacetime" all the...time). 

So our concept of time is intimately tied with our concept of space; space, and spacetime, and hence time all came into being (we think) at the big bang. When you squash spacetime back into whatever it's original state was, space and time as we understand them quit existing. And the very notion of "before" implies time, so saying "before the big bang" literally doesn't mean anything in a technical timey-wimey sense. 

There might be "something" "upstream" of the big bang but, whatever it is, our physics doesn't work there so even our basic concepts of space and time don't work there either.

It's like saying "What color was the wall before you build the house?". Without the house, you can't meaningfully talk about the color of the wall.