The default behaviour is to clear the cell. I was wrong to think the entire row was discarded.
In order to avoid warnings during the job execution, I used a workaroud with 2 steps in Recipe :
- a regexp (clear non matching cell for YYYYMMdd : ((19|20)\d\d)(0[1-9]|1[012])(0[1-9]|[12][0-9]|3[01]))
- then DSS (parse date)