The standardised calendar on Earth (Gregorian) is used to state the day, month, and year events happen on, since it shows how much time passes between events. This calendar is used on the Mushroom World, to show a sync in time, since Gregorian calendars would quickly become inaccurate on the Mushroom World as by the time a year has passed on Earth, it could only be a few days on the Mushroom World, which would massively misalign the standardised Mushroom World calendar, meaning there is no time dilation involved