I know what you all are thinking “He is Dutch, he must have been smoking some funny stuff” Rest assured nothing could be further from the truth.
30 February was actually a real date, well at least in Sweden.
February 30 was a real date in Sweden in 1712.
Instead of changing from the Julian calendar to the Gregorian calendar by omitting a block of consecutive days, as had been done in other countries, the Swedish Empire planned to change gradually by omitting all leap days from 1700 to 1740, inclusive. 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 that they did not omit leap days on the next two occasions and 1704 and 1708 kept leap years.
To avoid confusion and further mistakes, the Julian calendar was restored in 1712 by adding an extra leap day, thus giving that year the only known actual use of the 30th of February in a calendar.
That day corresponded to February 29 in the Julian calendar and to March 11 in the Gregorian calendar.
The Swedish conversion to the Gregorian calendar was finally accomplished in 1753, by omitting the last 11 days of February.