"time is relative" has a very particular meaning, it's not just a "proverb". It means (in the simpler special relativity) that how fast time seemingly progresse

"time is relative" has a very particular meaning, it's not just a "proverb". It means (in the simpler special relativity) that how fast time seemingly progresses is relative to the difference in velocities of two different inertial frames ("things that move at the same velocity and without external forces"). The only thing that special relativity assumes about flow of time in general is that within each of these frames, the speed of light is the same. This also means that you will always observe time progressing in any inertial frame, unless your relative velocity to it is the speed of light.