After accessing your query in Report Builder (Tool #100), select the option to Choose Files and select to add the DATELOOKUP table. To join this file, select 1 or 2 (matched records or matched with primary) and join by the compatible date fields. 

As an example, if your original file's date field is OPENDT and the format is MMDDYY layout, you would then match the OPENDT field with the matching field format in the DATELOOKUP table, DTMMDDYY. 
 
Once the DATELOOKUP table is linked in, you need to find and select the DTISODATE field for your report (under Choose Fields) and use that one to replace the original OPENDT.