Hi Howard, I can't really tell from your screenshot. But here is how I would troubleshoot this:
Confirm what is coming out of that particular field without any manipulation by removing the "addDays" completely.
based upon what value is, determine whether it is actually a date. If it is really a date ...it would show in a UTC format. You could also hover over the column in the mapper to determine the data type (text or date).
I suspect the data is literally blank or not actually a date. If it is not actually a date, then you'll need to use parseDate to get it to an actual date. Depending on the format you see when you look at the date, itself.