The February 30 or 30 February doesn't exist in both Julian and Gregorian calendar. The month February has only 28 days (29 days in a leap year).
February 30 was a real date in few calendars.
Early Julian calendarEdit
In 1700, Sweden tried to change their Julian calendar date to Gregorian calendar date by removing all leap years from 1700. However, the 1704 and 1708 became leap days by mistake. So Sweden added 2 leap days in February 1712 to match their dates with the Julian calendar. They changed to Gregorian calendar in 1753 by removing the 11 days from February.
Many sources claimed that from 1929 to 1940, Soviet union used a new calendar. It had 5-6 weeks and 30 days each month. Other 5 days were holidays in the calendar, just like the French Republican Calendar.