Thanks for all the replies, managed to get to a solution I was happy
with.

Much as I'd like to hanve thrown the toys out of my pram and insist
they get the dates  consistent before sending the xls files to me it's
not possible. The inconsistency seems to arise in third-party software
that pivots SQL queries and generates xls file automatically. Far
easier to resolve the issue  as it crosses my desk - which has now been
acomplished!