Here's how you can convert blank spaces into zero values in Excel:

1. Select the range of cells that you want to convert.
2. Press Ctrl+H to open the Find and Replace dialog box.
3. In the Find what field, type a single space character (" ").
4. In the Replace with field, type
   Formula:

   "0" 

   (without the quotes).
5. Click on the Options button to expand the dialog box.
6. Make sure that the Within: option is set to Sheet or Workbook, depending on your needs.
7. Check the Match entire cell contents box to ensure that only cells with a single space character are replaced.
8. Click on the Replace All button to replace all blank spaces with zeros.

That's it! Now all the blank spaces in your selected range will be replaced with zeros. This should allow you to perform calculations correctly even when there are no items sold during a certain time period. Let me know if you have any other questions!