February 30 occurs in some calendars, but not in the Gregorian calendar, where February contains only 28 or in a leap year, 29 days.

The Swedish Empire (which included Finland at the time) planned to change from the Julian calendar to the Gregorian calendar beginning in 1700 by omitting leap days for the next 40 years. Although the leap day was omitted in February 1700, the Great Northern War began later that year, diverting the attention of the Swedes from their calendar so they did not omit leap days on the next two occasions, causing 1704 and 1708 to remain leap years.

To avoid confusion and further mistakes, the Julian calendar was restored when, in 1712, one extra leap day was added, thus giving that year a 30th of February. That date corresponded to February 29 in the Julian calendar and to March 11 in the Gregorian calendar. The Swedish changeover to the Gregorian calendar was finally accomplished in 1753 by omitting the last 11 days of February.

Although many sources erroneously state that 30-day months were used in the Soviet Union for part or all of the period 1929–1940, in fact the Soviet calendar with 5-day and 6-day weeks was only used for planning the workdays and days of rest. The traditional calendar remained for everyday use—surviving physical calendars from that period only show the irregular months of the Gregorian calendar, including a 28- or 29-day February, so there was never a February 30.