If you have the desire to create a Raw Query for a Smart Folder that uses dates, here is how to do it. From the first drop down menu in the Smart Folder window, choose Other, then choose Raw Query, or whatever the appropriate tag is (check "Add to Favorites" if you use this a lot). Now enter an expression that compares a date metadata field to $time.today. For example:
kMDItemAttributeChangeDate >= $time.today(-7)
This will return all things whose attributes were created or changed in the last week (7 days). You could use this to create a Smart Folder which only shows files you've downloaded in the past seven days, as kMDItemAttributeChangeDate reflects when the file was added to the Spotlight database.
If you want to use a specific date, you need to do it in format that Spotlight understands. I would recommend first creating a Smart Folder with Last Opened, Exactly, and then put your date there. Save your folder and then open it in Text Edit. To do this, run Text Edit, then drag the folder to Text Edit's icon in the Dock. You can then search for the string RawQuery and browse through the line. You will see something like:
kMDItemFSContentChangeDate >= 137304000 && kMDItemFSContentChangeDate < 137390400
These are the system times on either end of the day you entered in the Smart Folder. You want to copy these to the Raw Query field, and edit them to reflect the attributes you wish to search on. The HTML entities need to be replaced, too:
>=; becomes >=
&& becomes &&
<=; becomes <=
You can play around with looking at other auto-generated queries in Text Edit, and then modify them using the Finder's interface to make custom queries.

