The easy way to do this (if you have good SQL skills!) would be to write a command (SQL Select statement) that does this filtering for you. When you use a command instead of tables, you need to create any parameters, such as your date params, in the Command Editor, NOT in the report! You then use the params in the where clause of the SQL.
Another way to do this would be to do the following:
1. Add a second copy of the table that contains the dates to the report. In the Database Expert, when you add a table that is already in the report, Crystal will give a warning and then add the table with "_1" on the end of the name. So, in your case you might have Inventory and Inventory_1.
2. Link from the part number in Inventory to the part number in Inventory_1. Make the link a Left Outer join.
3. Edit your selection criteria - you'll need to edit the formula instead of just selecting fields and values. Assuming a parameter called {?Start Date}, you'll add something like the following:
And ( IsNull({Inventory_1.DateField}) or {Inventory_1.DateField} < {?Start Date})
And IsNull({Inventory_1.PartNumber})
The first part of this limits the date range in the second copy of Inventory to dates prior to the start date of the range for the report. The second part says that you only want records in the Inventory table that don't have a corresponding record in the earlier date range.
-Dell