As long as the last-scanned field is consistently formatted, you should be able to convert it to a date type, then do a DateDiff to filter based on age.
Technet: CAST and CONVERThttp://msdn.microsoft.com/en-us/library/ms187928.aspxCONVERT(DateTime, field_containing_date_as_text, 101)
(I prefer to use ISO8601, but I'll assume most readers of this forum prefer American format and go with 101.)
Technet: DATEDIFFhttp://technet.microsoft.com/en-us/library/ms189794.aspxDateDiff( d,
CONVERT(DateTime, field_containing_date_as_text, 101),
GetDate() )
should give you the number of days between the last-scan-date and today, so
DateDiff( d,
CONVERT(DateTime, field_containing_date_as_text, 101),
GetDate() ) > 30
should filter for only those dates that are more than 30 days ago.