By any chance is the field of type text instead of number? Check this in Manage | Database | Fields.
When I enter postive and negative numbers into a number field, I find I can use a range with a negative number and it works. If it is of type text, it does not work.
It is a calculation field that finds the difference between two dates and returns a number like result. So i suppose it should be ok.
Yes, it should work--it did for me in the test I reported earlier here and a calculation that returns number (not text) should index identically as a data field of type number and thus should be searchable from a number range that includes a negative value.