This is updated every time you print a Query report to a printer.  It is updated when you use F5-View to see the results of the Query on your screen, or run the Query to “Display.”  For Queries that output to a database file, this date is NOT updated if you use the “Run” or “Run in Batch/Job Queue” feature to generate a file only.
 
Also, this date is cleared altogether whenever we install a new System i (for example, for online CU*Answers clients this occurred during June of 2013).  So if there is no date in this column, it means the Query was not “used” (as described above) since that time.