I am not associated with the research group. I tend to think that a group of paleo-climatology professors would remember that their country used to having different calendars.
Shouldn't a scientific paper making a specific scientific claim explain their methodology? Why should the audience presume that the method used to calculate the dates matches the implied trend? The calendar issue undermines their entire claim, and we're just supposed to give them the benefit of the doubt and trust that whatever they did was correct? Where's the scientific objectivism in that?