You want the angle you put into cos, and sin to be the same, and a value between 0, and 360. Use a variable and implement a counter to add to that variable.
Also you want to use the object that you want to rotate around as the x, and y.
Then set yourvariable using the variable + something *dt.
Note it will probably work if you go beyond 360, but you can always use %360 as a limiter.