As an alternative, what you can do is create a parameter of type Date , create calculated field for both of your data sources with regards to the created parameter , and then filtering the results by selecting only the "True" cases.
The only downside to this approach is parameters are not dynamic so wont be changing according to your data.
Hope this helps!!
Michelle, you wrote: "When we try to link the data sources by date, the date in the secondary datasource is no longer recognized as a date."
Can you describe that in more detail, and/or provide a packaged workbook? As one possibility, I'm guessing that the date in one data source could be a string data type and the other one a date datatype.
You also asked, "And how do you perform a calculation using two fields from different data sources?"
Create a calculated field in one of the data sources. In the calculated field dialog, in the drop-down under Fields you can choose the data source to pull fields from:
One thing to note is that when working with calculated fields and blending, all fields must be aggregate.