we may have a serious problem with the definition of time as that is currently defined using atomic clocks. And the rest of the SI is also rather relative because of this (the unit of length is derived from the unit of time by light speed).
Time is relative, we know this.
A second for you, whether measured using your trusty pocketwatch or the electron oscillation frequency in the fancy atomic clock taking up most of your office, is always a second, as c is constant (and so your metre is always exactly a metre etc.)
A second for another observer in a different gravitational potential or going at very different velocity through space will be different to your second though, and its by comparing the two that we can detect the distortions in space time that Einstein stated would be caused by gravity. However, for them, c is also constant, so their second will always be exactly one second and a metre exactly one metre to them.