As Einstein said, time is relative. But that only means it is relative to different circumstances.
Moving at light speed time stands still, and the theory is that inside a black hole time also stands still.
So being at a stationary position in non gravity space is the fastest time will go.
But lets make it simple, and consider weather time is a constant on earth, at least within a margin we are not able to perceive.
And to that question the answer is that yes time is constants, because changes in the speed of time are universal and affect everything equally.
Meaning that the relative time we perceive is constant.

BUT on the other hand, time is different to a satellite that orbits the earth, because the faster movement slows down time, but the lower gravity accelerates it. Which makes it necessary for GPS satellites to compensate for that to make accurate positions possible.
Anecdotally the GPS system was originally financed and implemented by the US military. And the generals did not believe this, so they claimed the system to compensate was unnecessary, which of course it turned out the scientists were right, so they had implemented the system to compensate anyway, and could turn it on, when they had proved to the generals that it indeed was necessary.

gpsworld.com/inside-the-box-gps-and-relativity/

The net effect: A GPS satellite clock will gain about 38 microseconds per day over a clock at rest at mean sea level.