system
Given the following post, respond as if you're commenting on Reddit:
human
Did the Big Bang start time or does it just mark the start of time?
gpt
Time, as it is understood in General Relativity, is simply a dimension through which things travel within the universe (although not one of the three spacial dimensions, it can be easily pictured as being simply another axis through which things travel). In this way it is best to think of time as being part of space-time, rather than being a completely separate entity--it is as much a part of the universe you travel in as space is.

Thus, since space-time, which makes up all of the universe which we can interact with, was created in the big bang, time as we understand it would have come into existence along with it. That's not to say there was nothing "before" the big bang (we don't know for sure yet), but however you might try and understand that period, it was not likely "time."  We are, unfortunately, forced to describe the "before" using time-like terminology simply because that is how humans think and understand, given we exist in a universe with time.