Time, as described by the theory of relativity, is indeed relative. This concept is supported by experimental evidence and has been confirmed through numerous experiments and observations.
The theory of relativity introduced the idea that time is not an absolute quantity, but rather depends on the relative motion between observers and the objects they are observing. This is known as time dilation. According to this theory, the passage of time is affected by the velocity and gravitational field of an object or observer.
In special relativity, time dilation occurs as objects move relative to each other at speeds approaching the speed of light. As an object's velocity increases, time appears to slow down for that object compared to a stationary observer. This effect has been confirmed by experiments, such as with atomic clocks on fast-moving airplanes and satellites.
In general relativity, the presence of a gravitational field also affects the passage of time. Clocks in stronger gravitational fields, such as those near massive objects like planets or black holes, run slower compared to clocks in weaker gravitational fields. This effect has been verified through experiments as well, such as the famous Hafele-Keating experiment.
As for your question about an "absolute measure" for time, it's important to understand that the concept of absolute zero in temperature is different from the relativity of time. Absolute zero is a temperature at which molecular motion theoretically stops completely. However, time is a fundamental dimension that is intertwined with the fabric of space. The theory of relativity provides a framework for understanding the relationship between space and time and how they are influenced by motion and gravitational fields.
The speed of light in a vacuum plays a fundamental role in the theory of relativity, but it does not serve as an absolute measure for time. Instead, it is a fundamental constant that relates the geometry of spacetime to the propagation of electromagnetic waves.
In summary, our current understanding of physics, supported by experimental evidence, indicates that time is relative rather than absolute. It is influenced by the relative motion between observers and the gravitational fields they are in, as described by the theory of relativity.