For some reason unknown to me, the designers of java.util.Calendar class have enumerated months of the year from 0 to 11 inclusive. So, January would be month zero, and December month 11. Very thoughtful of them. Or, maybe this is due to indexing of arrays in java.
So when you pass the Calendar.month constant through a to “public final int get(int field)” method of the java.util.Calendar class or its useful descendant java.util.GregorianCalendar class, note that you have to add one to the returned value to obtain an integer representing the calendar’s month in the way we humans expect a month number to be.