Well, it's all fanon to me to try and convert Gregorian to Mouvelian, and it's not going to be perfectly in line with GW1 because it never really has worked out right (Wintersday has always been bizarrely celebrated in the middle of real-life winter) even since the beginning. At some point, concessions have to be made for the calendar to be useful to us as roleplayers. Whatever fanon explanations people need to come up with to make the lore work - that the calendar has changed, that Wintersday as a holiday might have moved along a liturgical calendar or something, whatever it takes - don't really matter at the end of the day.
Rarely do I think fanon is useful to the community as a whole, but I think that THIS fanon - the calendar adjustments to sync up our make-believe and our real calendars - is more beneficial than it is harmful, overall, in spite of the lore troubles. No one is forced to use it, but I wish more people would.