>Maybe it is too complicated so that I don't know how to tell you correctly. The INDEX consistency error will be appeared when one of the following action I do:
>1. When I scroll up the cursor to the first data in the table (grid1), if I move the cursor 1 step up, the error will appear.
>2. Since the button's click event is a "Add" command, I click this button, an empty field will be added and the cursor is focus at this new data. If I move down the cursor 1 step, the error will appear too.

If you have an index on the date field, make sure it uses "machine" collating sequence. National collating sequences are a must for character fields in most of the languages, but they are a disaster when applied to other data types.