We don't know whether time really exist or not. It is the relative movement of matter, things, energy etc. that gives us the sense of passing of time. We know that the earth revolves around the sun in 24 hours (slightly early) because our watches and clock show us that 24 hours have been elapsed but if the earth begin to move much faster or slower and simultaneously everything in the universe begin to move similarly faster or slower then we cannot realize that the speed of the earth has increased or decreased.
The enormous objects like black hole can alter the speed of time. If someone could survive near a blackhole, he will see everything moving very fast while a person on earth will see him as stationery because the relative speed of time is different in both the places.
I realized that when I was a child time used to move very slowly but when I grow up time is moving very fast for me. I don't find 24 hours enough for me in one day.
So, time is an extraordinary thing in this universe.
No, you would notice, because we set 1 second as a certain amount of time relative to movement in existence. If the earth would speed up, 1 second would still be one second for us, but we would have less time to spin around for 1 day, so the day would change length. 1 second is arbitrary, based on a division of the day. The day is what determines our measure of hours, minutes and seconds.
I wish I had more than 24h too hehe.
Yes, 1 second is a time in which the atom of Cesium completes 9,192,631,770 frequencies. But what will happen if Cesium atom complete that resonance in just half the time and so the other atoms? Will be able to realize it? No, never.