How did Sweden end up with a February 30 in 1712?

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Kalendarium (“Calendar”) by Regiomontanus — calendar First complete printed title page for the *Kalendarium* (“Calendar”) by Regiomontanus, 1476.

In 1699, Sweden planned to transition to the Gregorian calendar by omitting leap days over a 40-year period. While the leap day was successfully skipped in 1700, the government’s focus on the Second Northern War led to the accidental retention of leap years in 1704 and 1708.1

This resulted in a unique Swedish calendar that was 10 days behind the Gregorian system and one day ahead of the Julian calendar. To rectify this inconsistency and return to the Julian system, King Charles XII ordered the addition of a second leap day in 1712, creating the date February 30. Sweden eventually adopted the Gregorian calendar in 1753 by skipping 11 days in February.1