If you are trying to determine the number of new constituents for the past month, the best thing to do would be to run a query based on Creation Date. 
You can do that by doing the following: 
  1. Go to Data Management -> Query. 
  2. Click Create a new Query. 
  3. Give the Query a Name and Description. 
  4. Click Save. 
  5. On the Query Builder Tab, choose "System" from the Field Type drop-down, and "Creation Time" from the Field drop-down. 
  6. Click Add Field Clause. 
  7. Choose "equals" from the first drop-down. 
  8. Choose either Fixed or Relative and fill in the date ranges you want. 
  9. Click Save. 
  10. Click Save Query. 
  11. Click Run Query. 
Once you have run the query, on the Results tab, you can click Use Query to either produce a Mail Merge, which will list the Constituents pulled in by the query, or you can add the results to a group.