I know we were just told that Discovery is a true prequel to TOS, but I think by now we've all seen the trailer and realized that that can't really be true. In fact, when Bryan Fuller initially said it was set in the Prime Timeline he also said that it could be in either one. But a direct comparison shows which timeline it most closely matches:
And while not a direct comparison, look at the design on this helmet compared to the Klingon architecture in STID:
I don't see anything in the trailer that resembles TOS, it's all clearly based on the aesthetic of the new movies (just look at all the lens flares.) So until we get a real indication to the contrary, I'm going to say that Discovery is set in the Kelvin Timeline.








And while not a direct comparison, look at the design on this helmet compared to the Klingon architecture in STID:

I don't see anything in the trailer that resembles TOS, it's all clearly based on the aesthetic of the new movies (just look at all the lens flares.) So until we get a real indication to the contrary, I'm going to say that Discovery is set in the Kelvin Timeline.