You should learn to read the java API. This has all the documentation for the predefined classes, including the Calendar class. It states in there:
public static final int MONTH
Field number for get and set indicating the month. This is a calendar-specific value. The first month of the year in the Gregorian and Julian calendars is JANUARY which is 0; the last depends on the number of months in a year.
(emphasis added by me)
Usually, when I'm coding, I always have that page open to check and see what things do.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors