Thread: Finding a Rate of Change?

I am extremely lost for this problem and I am in need of some help please. The function given is D(t) = 12 - 3cos(0.0172 (t + 11)). The question asks "How rapidly are we gaining daylight 90 days after the minimum occurs?" The minimum was at 21 days so I am using a t = 111. Does anybody know how to calculate this as a rate of change? I just need to change the formula so it is a rate of change. Any help would be GREATLY appreciated.