In some ways I hate getting roped into questions like this (but cannot resist it) as they are relative to the perception of the subject. The stuff that gets taught in school is what I shall elucidate upon, thus precluding the personal view. To resolve
your question, I will introduce a definition of "relativity" from the compact version of the
• noun1 the absence of standards of absolute and universal application.
2 Physics a description of matter, energy, space, and time according to Einstein’s theories based on the importance of relative motion and the principle that the speed of light is constant for all observers.
In a nutshell, time is NOT a constant, "The Speed of light is!".
That is the answer to your question.
When you look at the Moon or Jupiter you do not see it / them as 20:58 GMT (the time now presently here in England), but as the time the light has taken to travel from either celestial bodies. Time merely helps us all to get to work at the same time, or attack
enemy x at time y. any celestial body outside the solar system has a different time-frame as the sun is no longer it's determinant.
I like to think of Douglas Adams's Hitchhikers Guide to the Galaxy!