Just discovered that the "Object format" defined for the date in the Universe, has bearing on this problem.
* Dates which have Object Format assigned of "Date/Time" "dd/mm/yyyy" are the ones which won't "Save as PDF" correctly under 4.1.
* Dates which have no Object Format are Ok.
I am not sure how and why this has any bearing on whether or not the dates can be exported "as PDF", but it seems like (that under 4.1 at least) it does. The same behavior was not experienced under 3.1 (not for dates that had ObjectFormat added, nor for dates that had no object format assigned).