Relative

 Below, a couple having a hard time of it. It takes them ever longer to understand 

each other:

                                                             

From the very nice explanation of relativity:

https://www.science.org.au/curious/space-time/gravity#:~:text=Gravity%20bends%20light&text=Light%20travels%20through%20spacetime%2C%20which,of%20light%20caused%20by%20gravity%20.

With respect to gravity, everyone agress that the clock near a massive object is slower.

                                                                          

                                              Wikipedia

GPS systems have to take both phenomena into account.

                                                                *     *     *