My favorite calendar disaster was the Swedish calendar[1] between 1700 and 1753.
They had a somewhat (?) reasonable plan:
In November 1699, the Government of Sweden decided that, rather than adopt the Gregorian calendar outright, it would gradually approach it over a 40-year period. The plan was to skip all leap days in the period 1700 to 1740. Every fourth year, the gap between the Swedish calendar and the Gregorian would reduce by one day, until they finally lined up in 1740. In the meantime, this calendar would not be in line with either of the major alternative calendars and the differences would change every four years.
Then war started and they stopped, after starting the process, so they were on their own calendar that no one else in the world used.
Then the king decided it was a bad idea, so reverted to the Julian calendar. Then they decided to jump to the Gregorian outright.
There's a bunch of other details, like Easter being calculated by one calendar but Easter Sunday by the other.
Oh yeah, and they had February 30 one year[2] (which was Feb 29 in a Julian calendar or March 11 Gregorian).
And in 1753 they finally sorted it all out by skipping all the days between February 17 and March 1.
They had a somewhat (?) reasonable plan:
In November 1699, the Government of Sweden decided that, rather than adopt the Gregorian calendar outright, it would gradually approach it over a 40-year period. The plan was to skip all leap days in the period 1700 to 1740. Every fourth year, the gap between the Swedish calendar and the Gregorian would reduce by one day, until they finally lined up in 1740. In the meantime, this calendar would not be in line with either of the major alternative calendars and the differences would change every four years.
Then war started and they stopped, after starting the process, so they were on their own calendar that no one else in the world used.
Then the king decided it was a bad idea, so reverted to the Julian calendar. Then they decided to jump to the Gregorian outright.
There's a bunch of other details, like Easter being calculated by one calendar but Easter Sunday by the other.
Oh yeah, and they had February 30 one year[2] (which was Feb 29 in a Julian calendar or March 11 Gregorian).
And in 1753 they finally sorted it all out by skipping all the days between February 17 and March 1.
Good luck writing billing software for that.
[1] https://en.wikipedia.org/wiki/Swedish_calendar
[2] https://en.wikipedia.org/wiki/List_of_non-standard_dates#Swe...