Thanks for the suggestion.
That works, in that it displays correctly in a MyRootsMagic website. Perhaps explicitly stating the date range is more 'correct' and GEDCOM-standard.
("abt Nov 1867" works as well, and is considerably shorter!)
However I have a lot of quarterly dates in my database, and I do not want to have to replace them all. Quarterly dates also have a particular meaning/implication for GRO birth registrations in England, which I would like to preserve.
I'm assuming this problem also means that any date which does not pass RootsMagic validation will not display in the website. So if I enter a custom/non-standard date, it will be replaced in the website display with "unrecognized date format." I think the published site should display what is in the date field without prejudice to validation.