I usually give a range of cooking times because all ovens cook differently. Weather and altitude plays a part too! Be sure you have an oven thermometer to test the heat in your oven. The dial might say one thing but the inside of the oven might read differently. If your recipe isn't done when the time is up, maybe your oven cooks slower than mine. Just keep cooking. I always check my food halfway through baking to see if it needs to be rotated (or if it's cooking faster). If something is browning too much, cover it with foil during baking.