Formula When Date Entered in One Cell Another Cell Increases or Decreases by 1

I am trying to find a formula that when a date is entered into a specific cell that falls during a particular week (i.e. 5/17/2015 thru 5/23/2015), another specific cell increases or decreases by 1. I have copied below the tables I am trying to formulate. I am trying to make it so that when a date is entered under the "Contract Start" on the bottom list , the cell "Starts" on the corresponding week ending date increases by 1 and vice versa for the "Contract End" to decrease by 1. Any assistance would be greatly appreciated. Thank you for your help.