I don’t think that it is good idea to search in the string - there are lot of posibilities to make complex filters.
I think that if you really don’t want to get such error (which is not very understandable for user), you have to add Starting Date and Ending Date fields in to request form and test if user have entered them.
Or you can apply the filter to Date table “Starting date” field and try to find first entry. If it’s not 01/01/0000 then min. range exists.
I did not want to use the StartDate EndDate in the Options as it seems a bit crude. I am sure that it must be possible to find if the filter is “…311207” and if so, enter a MinDate that is say -3M the MaxDate.