Time can be measured universally, but due to gravitational time dilation, different clocks located at different gravitational potentials will measure time differently. Gravitational time dilation is a consequence of general relativity, which states that the gravitational field affects the flow of time.
In regions of stronger gravitational fields, such as near massive objects like stars or black holes, time runs slower compared to regions with weaker gravitational fields. This means that clocks located in stronger gravitational fields tick slower than clocks in weaker fields when observed from a common reference frame.
The effect of gravitational time dilation has been observed and measured in various experiments and observations, including precision atomic clocks. For example, clocks placed at different altitudes on Earth's surface will measure slightly different times due to the variation in gravitational field strength at those locations.
Despite gravitational time dilation, it is still possible to establish a notion of coordinated universal time (UTC) by using suitable conventions and synchronization methods. Coordinated universal time is a global time scale that takes into account relativistic effects, including the gravitational time dilation caused by variations in Earth's gravitational field.
In practice, timekeeping systems, such as atomic clocks, are used to establish a standardized and synchronized time scale that can be widely adopted. Corrections are applied to account for relativistic effects, including gravitational time dilation, to maintain consistency and accuracy across different locations.
Therefore, while gravitational time dilation introduces variations in the flow of time at different gravitational potentials, efforts are made to establish a universally agreed-upon time scale that takes into account these relativistic effects.