Wherever there is matter there is gravity & time because matter is made of atoms.
Atoms create their own gravitational pull due to the electrons spinning around the nucleus (protons & neutrons). Atoms make up matter.
The more matter you have grouped together ( ie a star ) the more gravitational pull it has on other object (ie a planet)
To truly understand time, gravity & inertia you need to look at the subatomic level and dark matter. Ready to have your mind blown?