The return value is always 0 because the System.Globalization.JapaneseCalendar type does not support the notion of a leap month.
In a calendar that supports the notion of a leap month, the leap month can occur either after a particular month or after any month in a year. For example, the EastAsianLunisolarCalendar.GetMonth(DateTime) method returns a number between 1 and 13 that indicates the month associated with a specified date. If there is a leap month between the eighth and ninth months of the year, the EastAsianLunisolarCalendar.GetMonth(DateTime) method returns 8 for the eighth month, 9 for the leap eighth month, and 10 for the ninth month.