That's a big error in that article.
It should be 30 december 1899.
My guess is that somebody misread 12/30/1899 as 12 december (which it is not)
but because in lots of Europe it is written as 30/12/1899 they thought the first digit was the day.
See the manual of Delphi.The integral part of a System::TDateTime value is the number of days that have passed since 12/30/1899. The fractional part of a System::TDateTime value is the time of day.
Originally it should have been 31th but because of some misconception about 1900 being a leap-year (which it is not) it eventuelly became 30th december.
See explanation here:
http://www.delphibasics.co.uk/RTL.asp?Name=TDateTime