When I try searching for objects in Inmagic Presto 3.5 within a date range I get redirected to the "We are sorry" error page. Here's an example search making use of the built-in DateCreated field on the built-in Profile object:
Profile.DateCreated=[20101201 TO 20101225]
The date format (yyyymmdd) is taken from the documentation. I've also tried ddmmyyyy and mmddyyyy.
The example in the documentation is wrong - the date needs the time part adding so the format should be: yyyymmddhhmmss. For example:
Profile.DateCreated=[20101201000000 TO 20101225000000]
If an advanced search page is configured to include a date field then the UI can be used to build a valid search query. The correct format can then be seen when the search is executed.