system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Time

Okay, I get the basics of time and how it works. What I'm wondering is why is it 'running' at exactly the speed that it is and how does that work? Has it always been running at this speed? What is there to indicate that time even exists?
gpt
Time is a human measurement of the environment that we live through. We measure it by an arbitrary scale that we invented/derived. 

Basically though time is a measurement of change. Time as we think of it has existed from the big bang/creating of the universe. 

Time does not "run" at the same speed everywhere as far as we can tell. It is confirmed that time passes differently at higher/lower gravity, or at high or low traveling speed. Our satellites in outer space literally have clocks that are running at "different" speeds than the ones on earth.

Time only goes in one direction because our consciousness only goes in one direction. We remember the past, but not the future. We can more or less easily look into the past, and know events happened in a specific order.