因为 Calendar.MONTH 是从零开始的.为什么?

Because Calendar.MONTH is ZERO based. Why?

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get 和 set 的字段编号，表示月份.这是一个特定于日历的值.公历一年的第一个月儒略历是 JANUARY，即 0；最后取决于一年中的月数.

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.