system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How can time have a starting point?

Did time just started happening? What does it even mean for time to just start?
gpt
Imagine nothing. If that bends the mind too much, imagine a metal sphere infinitely small suspended in space with absolutely nothing around it. No air, no light, no movement, force, anything. 

If you left that sphere alone, nothing would change. It wouldn’t move, it wouldn’t rust, etc. Essentially, it is unaffected by time. 

Now imagine you are the sphere. For you, time doesn’t exist. A million bajillion years is the same as a millisecond, as nothing changes at all. Time could pass for an eternity and more, and you wouldn’t notice.

Now imagine if the ball suddenly explodes. Now time is important. Things are moving at certain times, things are moving because you caused it, therefore they have an effect, and they carry out that effect at certain times at certain speeds. You exploded, so now you have space to travel, and it takes time for you to travel that space. 

That is the creation of time. Space and time are inextricably linked, as in order to have time, you must have some sort of movement to due in that time. Since before the Big Bang there was no space to move, it being a singularity, there was no time either.