Thank you for your post.
Can you verify the field is defined as Date? If it is defined as Date, first turn off the indexing for the field, and then perform the find again, as this will reindex the field.
And perhaps you are not performing a find? If this is a portal filter or used in a match field for a relationship, this type of range notation is not correct syntax for these options nor can it be used in an ExecuteSQL function call.
I figured out that I had the filed as a Text instead of a Date. Once I changed that, the search worked like i needed it too. Thanks