Inertial reference frames are typically defined as frames of reference in which an object at rest remains at rest or an object in uniform motion continues to move with a constant velocity. In these frames, the laws of physics, including the principle of inertia, hold true.
However, it's important to note that time is still a variable in inertial reference frames. In special relativity, it is understood that time is not absolute but rather relative to the observer's frame of reference. Different observers in relative motion will measure the passage of time differently.
According to special relativity, when two observers are in relative motion, their measurements of time, as well as other physical quantities such as length and mass, will be different. This difference is accounted for by the phenomenon of time dilation. Time dilation means that the rate at which time passes for an object in motion is slower compared to an object at rest relative to the observer.
So, while an inertial reference frame provides a useful framework for describing the motion of objects with respect to each other, time remains a variable that is affected by relative motion and can result in measurable differences between observers in different inertial frames.