The quote below was posted in the DEVONagent section, but similar questions can be raised about how to make use of dates for searching and analyzing information in DEVONthink.
DEVONthink will let one “slice and dice” data in a lot of ways. DT’s Boolean search operators are not as powerful as those in DEVONagent (but watch for future updates!), but if you think out the logic of a query that you want to make, DT can do very complex searches and data manipulation.
Remember, of course, that DT is a freeform database (although the current alpha version of DT Pro allows tables and records, that hasn’t been released yet). Also remember that dates can be written in many ways, which can complicate things.
DT always date stamps items with their creation date and their modification date.
Here’s a simple example. Suppose some benevolent person pays me ten cents for each item I add to my database. Now suppose a question arises as to how many items I added from May 3, 2003 to June 14, 2003.
That turns out to be easy.
[1] Create a new group, “May-June 2003” anywhere you wish.
[2] Open the Search tool. Set it to search Comments, Global and enter the search term 2003.
[3] Sort the hit list by Date (if the header is Modified, click to change to Date).
[4] Select the first item for May 3, 2003. Hold the Shift key down and navigate to the last item for June 14, 2003, and click on that item. All the relevant hits are now selected.
[5] With the cursor over the highlighted items, Control-Click to bring up the contextual menu, and choose Replicate to > May-June 2003.
[6] Now look at the receiving group. There are 172 items in it (that’s low nowadays – I add that many every day or two). OK, I can verify a bill for $17.20.
[7] While I’ve got that group of items I could ask other questions about it. For example, do a search just for that group and ask whether the term “Einstein” occurs (2, but neither was Albert). Or, suppose I can’t charge for PDF files added to the database. Open the group in List view and sort by type. OK, no PDF files, so the billing stands.
[8] Now, if I wish (and I probably do), I can delete the “May-June 2003” group. No harm done, since the contents were merely replicates of existing files.
Think about the logical power of being able to temporarily "break out" items for analysis in a variety of ways. 
Here’s another example that might generate useful ideas. Suppose I keep a client list, and each time there’s activity with a client (a call, for example) I add that information to the client’s file – which results in an updated Modification Date. Just by opening the client list group in list view, and sorting by Modification Date, I can quickly review which clients haven’t been contacted recently. Or, by sorting by Size I can tell which files have the most “activity” (as defined by notes added to the file). There’s an almost endless series of tricks that could be used to analyze the data and plan for action.
Of course, with tables and records, DT Pro will allow addition of record fields of the user’s choice in a database. Even more power!
Update Monday, September 13, 2004 6:57:42 PM: Oops! The command “Replicate to” isn’t in the current release of DT PE, but is in a pre-release version of DT Pro. I’m pretty sure it will be available in DT PE 1.9. Obviously, it’s an enormously useful feature.
