To create this custom field, go to:
  1.  Setup, App Setup, Customize, Accounts or Contacts (based on the record you want the field on), Fields
  2. Scroll down to the Account Custom Fields and Relationships section and click New
  3. Choose Roll Up Field, click Next
  4. Enter a name for the field, click Next
  5. Choose Opportunities from the Summarized Object dropdown
  6. In Select Rollup Type, choose Sum
  7. In the Field to Aggregate dropdown, choose Amount
  8. Under Filter Criteria, choose "Only records should certain criteria be met ... "
  9. In fields, you'll create two:
    • Close date greater or equal to mm/dd/yy you choose
    • Close date less or equal to mm/dd/yy you choose
  10. Click Next
  11. Click Next
  12. Click Save
You can now go to a record and click the recalculate summaries button to populate this rollup field.