Time is invariant irrespective of frame of references.

Time is invariant means, it's succession remains unchanged irrespective of transformation applied on it. And it is universally acceptable.

A clock is not invariant means since it's mechanism subjects to various distortions, it cannot maintain invariant succession of time unless periodic adjustments applied in it to make it run as per the invariant succession of time.

When any made up reference frames are introduced within the universal frame of reference, then those introduced reference frames too ought to abide by the universal frame of reference, as no eventual frame of reference can be introduced beyond the universal frame of reference.

And time is invariant within universal frame of reference also mean time within all introduced frame of references would abide by the time in universal frame of reference, irrespective of those introduced frames of references are in relative motion or not.

The concept of universal time is always invariant so is any form of times in all reference frames.