Standard calendar cannot be treated as a Proleptic Gregorian for dates 1582-01-01
More context here: ancil/ticket #1159

UM F03 more recently clarified that the calendar it expects isn't Standard but a Proleptic Gregorian. Not only is iris pp save ignoring time coordinates with proleptic_gregorian calendars entirely, it also allows saving Standard dates prior to 1582-01-01 (as though they are proleptic gregorian dates when they are not).