Delta Field for Generic Extractor

I am in a fix. I have a Generic extractor on a transparent table. I am using a date field as delta specific field. This field is grayed out in RSA6 & does not allow me to check it for selection. But its not hidden. The problem is since I can't select the check for selection, I can't do selective init? There is lot of data for this extractor & I need that date field to do selective init & then do delta. Is it ok to use any other field for selection & still