Self-made fogey Michael Gove used the Roman number for the month in dating his party leader nomination papers.
According to Wikipedia, this mixing of Roman and Arabic numerals largely only holds today in Italy, Russia, Bulgaria, Croatia, Portugal, Romania and Serbia. It may also, I'm told, be favoured in fee-paying British schools. A current publisher's style guide says "Using Roman numerals for publication dates is largely an affectation of the 21st century and is not advised. Still, it might be considered for a book to which the designers or editors wish to grant a special aura of authority or importance."
The mixture was used by Marc Chagall.
Roman numerals are also occasionally favoured by style leaders such as Beyonce and David Beckham when inking their bodies with needles.