Not great, but nothing is worse than date calculations with the Swedish calendar between 1699 and 1753. They have a leap year with no Feb 29, a year with 30 days in February and 367 days in the year, 10 years where they used a globally unique calendar and a year where 1 March was the day after 17 February.

Quoting the entire thing:

> 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.

> In accordance with the plan, 29 February was omitted in 1700, but the Great Northern War stopped any further omissions from being made in the following years.

> In January 1711, King Charles XII declared that Sweden would abandon the calendar, which was not in use by any other nation, in favour of a return to the older Julian calendar. An extra day was added to February in the leap year of 1712, thus giving the month a unique 30-day (30 February) and the year a 367-day length.

> In 1753, one year later than England and its colonies, Sweden introduced the Gregorian calendar. The leap of 11 days was accomplished in one step, with 17 February being followed by 1 March

https://en.wikipedia.org/wiki/Swedish_calendar#Solar_calenda...