Lets correct some things.
The speed of light can be manipulated.
Where did you get this information from? The speed of light can not be manipulated but what you might be referring to, is that the speed of light depends on the medium the light travels through.
Gravitational time running at a steady rate.
The earths rotation is measured by the IERS and if you take a look at their data you will see that the earths rotation is not constant. Therefore is the gravitational time not running at a steady rate.