There still remains, for me at least the puzzle that unless I set Kmail to display the FULL headers of the digest messages from FSFE, the 'Long Headers' date everything from FSFE UK Thu Jan 1 00:59:59 1970.

I have however carried out an experiment 'Standard Headers', 'Brief Headers' and 'Fancy Headers' don't display the date on the email, (the digest version), at all. Having changed the headers there seems to be no alteration to the date displayed in the folder.