Validation of Time Dilation

The theory of relativity represents that time runs slower wherever gravity is strongest, and this is because gravity curves space-time. This phenomenon is known as time dilation. According to this, gravity's effect measures the amount of time that has elapsed between two events by observers at different distances from a gravitational mass. In accordance with the theory of relativity, time dilation is slowing of time that occurs in a system in motion relative to an outside observer and that becomes apparent especially as the speed of the system approaches that of light.

As per interpretation of static universe and before 1929, time had no beginning. Albert Einstein proposed a temporally infinite but spatially finite model as his preferred cosmology. Later it was concluded that the universe has not existed forever, rather, the universe, and time itself, had a beginning in the Big Bang.
However, now we know the definition of time is, the indefinite continued progress of existence and events in the past, present, and future regarded as a whole.
Therefore, if we interpret that the 'indefinite continued progress of existence and events' as time itself, then naturally questions would arise, "what is the unit of time, and name of the scale with respect to what time is measured?"

We know that 'second' is the unit of time.

We also know that time is the measurable period during which an action, process, or condition exists or continues. A nonspatial continuum, that is measured in terms of events which succeed one another from past through present to future. From its own nature time flows equably without regard to anything external, and by another name is called duration. It does not depend upon physical events, but are a backdrop within which physical phenomena occur.

Time is a reference frame in which such intervals are measured. Where a scale of measurement refers to ways in which variables are defined and categorized. Such a scale of measurement necessarily needs to be uniform in nature, fixed and not variable. Therefore, time itself is a scale of measurement with respect to this measurement of all variables - events and existences are determined. Time, as a scale of measurement is uniform and fixed, and cannot be either speeded up or slowed down.