Why Sweden put February 30 on the calendar
In 1700, Sweden tried to move gradually from the Julian calendar, which adds a leap day every four years, to the Gregorian calendar, which skips leap days in most century years. By then, the two calendars differed by 10 days. Sweden’s plan was to omit leap days over time, narrowing the gap without suddenly deleting a run of dates.
The first step was to leave out February 29 in 1700. That made Sweden’s calendar one day out of step with the Julian calendar. But a gradual change required skipping later leap days, too. Sweden did not keep to the plan: it observed leap days in 1704 and 1708, leaving its calendar neither fully Julian nor steadily moving toward Gregorian.
Two extra days to undo the experiment
In 1712, Sweden chose to return to the Julian calendar. Because the failed transition had left it one day behind that calendar, simply restoring the usual February 29 would not be enough. The Julian calendar called for a leap day that year, so Sweden’s February had both February 29 and February 30. The extra day brought the dates back into alignment with the Julian system.
Sweden later adopted the Gregorian calendar outright. In 1753, it made the change by moving from February 17 directly to March 1, skipping 11 dates. The February 30 of 1712 was not a new way of counting time; it was a one-time repair to a calendar change that had been abandoned.
