There is a universal time besides GMT called UTC, but you always need to display locale time to users because it is the only time they understand.

That means it’s not a 100% instant response as some people are sleeping (can’t reply at the moment) but reply when you’re asleep.

If you go outside you’ll notice there’s this thing called the sky. It gets dark sometimes. Sometimes its bright. That’s because of the sun. People like to sleep when it is dark. When the sun is on the other side of the world its day time for them. But night time for you. So you are sleeping and they are awake. Even if we just used universal time for everyone, the sun would still do the same thing. And people would still be asleep while you are awake.

What time it is and when people sleep do not need to be related. We could have a universal time that would mean some people’s day starts at 8 while others starts at 22, but people would still sleep at night. Nighttime would simply not be related to the same numbers in every region under such a system.

Why does it matter that everyone sees a different time on their clock? People on the east coast will be asleep whether you see 04:00 EDT or 08:00 UTC. Timezones give you an easy way to predict what part of the day it is in some place without physically being there.