Actually, if you care to head on over to the Wikipedia page for time dilation-
https://en.wikipedia.org/wiki/Time_dilation#Combined_effect_of_velocity_and_gravitational_time_dilation You'll see that you are indeed correct- time dilation can be caused by both motion and mass- but the end result is a combined effect of the two- without one, the other cannot exist. It even gives the "combined effect" equation on the page, which interestingly is based off of the generally accepted model of our solar system and is incredibly accurate, which in itself gives fantastic evidence for a round-earth model. Of course Round Earthers would argue that uniform acceleration would never reach the speed of light- as long as gravity is accepted to exist, then this holds completely true as you will always be under the effect of gravitation from a celestial body of some kind, and gravity does indeed exist.
I'm not too sure where you got your equation from- it seems like it may be a simplified version of the equation to find the velocity constant in the equation, rather than the overall effect, though that's just my assumption through comparison with the wiki site- but it is indeed a combination effect.