> "Turns out this day won't exist for you" is never something I'd have considered a possibility.
When countries switched from the Julian to the Gregorian calendar this happened everywhere. Entire counties skipping weeks in their calendars. Historical documents referring to the same event but using different dates, because they were using different calendars. Fun times. And of course not every country switched at the same time.
The Wikipedia page about this is pretty interesting [0] Some quotes from that page:
> In Russia, the Gregorian calendar was accepted after the October Revolution. On 24 January 1918 the Council of People's Commissars issued a decree that Wednesday, 31 January 1918, was to be followed by Thursday, 14 February 1918, thus dropping 13 days from the calendar.
> Catholic countries such as the Polish–Lithuanian Commonwealth adopted the "new style" (N.S.) Gregorian calendar in 1582 (switched back in 1795 after Third Partition of Poland)
And if you think that is bad, the Roman calendar that was replaced by the Julian calendar had a year consisting of 304 days and when things got too much misaligned with the seasons some extra days would be added [1]
> The system is usually said to have left the remaining 50 odd days of the year as an unorganized "winter", although Licinius Macer's lost history apparently stated the earliest Roman calendar employed intercalation instead[7][8] and Macrobius claims the 10 month calendar was allowed to shift until the summer and winter months were completely misplaced, at which time additional days belonging to no month were simply inserted into the calendar until it seemed things were restored to their proper place.
Good thing computers weren't around back then.
[0] https://en.wikipedia.org/wiki/Adoption_of_the_Gregorian_cale...
[1] https://en.wikipedia.org/wiki/Roman_calendar#Legendary_10_mo...