# DSolve returns unevaluated(I need a closed or numerical solution for this system and plot solution)

I am trying to find a solution to this system of differential equations but the program gives the same output without any messages. I would like help. Please find the Mathematica code posted here. a and d are Constants, $$I=\sqrt{-1}$$

S1 = I y' == Cos[a t] Exp[- I d t] x;

S2 = I x' == Cos[a t] Exp[ I d t] y;

S = {S1, S2};

DSolve[S, {y, x}, t]


I haven't found a way to get DSolve to crack it, so turn to NDSolve
a=1;d=2;