Android calendar 更改日期操作

在项目中，需要修改日历的时间，之前通过下面的代码，无论如何修改，得到的日期都是同一个值：

Calendar startDate = Calendar.getInstance();

Calendar endDate =  Calendar.getInstance();

startDate.setTime(mStartDate);
endDate.setTime(mEndDate);

switch (mFreq) {


case RepeatSelectionActivity.PERIOD_DAY:
startDate.add(Calendar.DAY_OF_YEAR, 1);
endDate.add(Calendar.DAY_OF_YEAR, 1);
break;
case RepeatSelectionActivity.PERIOD_WEEK:
startDate.add(Calendar.WEEK_OF_YEAR, 1);
endDate.add(Calendar.WEEK_OF_YEAR, 1);
break;
case RepeatSelectionActivity.PERIOD_MONTH:
startDate.add(Calendar.MONTH, 1);
endDate.add(Calendar.MONTH, 1);
break;
case RepeatSelectionActivity.PERIOD_YEAR:
startDate.add(Calendar.YEAR, 1);
endDate.add(Calendar.YEAR, 1);
break;

}

修改为下面的代码即可成功修改日期：

Calendar startDate = new GregorianCalendar();
Calendar endDate = new GregorianCalendar();