This can be done by following these instructions:

  1. Select the report fields and set comparison to previous period of same length.
  2. Run the report.
  3. Unhide a worksheet called "querystorage". There you'll see the parameters for each of your existing reports. Find the column that contains the report you just ran.
  4. In the the querystorage sheet, in the row labeled "Comparison", you should see the value "previous". Change this value to "custom", and in the two rows below labeled "Comparison start date" and "Comparison end date", type the date range you wish to compare against. See image below:

  5. Go to the report and press the Refresh button. The figures will now be compared against the custom date range.