Optional Date Parameters and ISNULL

In a graphical Calculation View, I've created a single input parameter. P_START_DATE

The notion is that if the user does not supply a date, the beginning of the period should be used, otherwise the user_specified_date should be used to filter table.

No matter what I do, $$P_START_DATE$$ evaluates to a nonnull value in any runtime expression but when copied to a calculated column, it displays as <NULL>.

I've tried setting the default to constant NULL, expression null.

I've tried setting the value of a calculated column L_START_DATE with nullif and if(isnull()). Oddly, this column will display as <NULL> in the preview.

Puzzling.... But if this is the case, what's the point of a nonmandatory prompt?

Bug? SPS11