Yes, there is a function in Excel that can help you calculate the weekly return of a stock. The function is called "WEEKNUM" and it can be used to determine the week number of a specific date. Here are the steps to calculate the weekly return:

1. Enter the closing prices of the stock for each week in a column in Excel.
2. In the next column, use the "WEEKNUM" function to determine the week number of each date. For example, if the first date is in cell A2, you would enter
   Formula:

   "=WEEKNUM(A2)" 

   in the adjacent cell in column B2.
3. In the next column, calculate the weekly return by subtracting the closing price of the previous week from the closing price of the current week, and then dividing the result by the closing price of the previous week. For example, if the closing price for week 1 is in cell C2 and the closing price for week 2 is in cell C3, you would enter
   Formula:

   "=((C3-C2)/C2)" 

   in the adjacent cell in column D3.
4. Copy the formula in step 3 down the column to calculate the weekly return for each week.

By following these steps, you should be able to calculate the weekly return of the closing price of the stock for 15 weeks. Good luck with your project!