In the Admin area backend, go to the "Menu" area and create a new category called "Specials" (or Lunch Specials, Dinner Specials etc..). This would be just like any regular category, except you can give the category specific timing for when those prices apply.
By enabling the "Time Restriction" you can set a specific time for when the items in this category can be ordered. For example, if you set the time for 11am to 3pm. This category will be displayed at all times, but customers cannot order the items in this category outside of the 11am to 3pm time frame. Their order must be for 11am to 3pm. You can add as many items as you want within this category.