Cooking times given in recipes are only an indication of how long something will take to cook. The temperature of the food as it goes into the pot or oven, the calibration of the oven, the condition of the burners, the type of fuel used (propane or city gas or electric), even the altitude above sea level—all of these cause variations in cooking times. So use the given times, but for perfect results, start to check the dish up to ten minutes before the recipe says it will be ready, and be prepared to go ten minutes more.