java的日期时间操作

一，指定时间上的分钟操作

        public long addMinute(Date date, int minute){
		Calendar cal = Calendar.getInstance();
		cal.setTime(date);
		cal.add(Calendar.MINUTE, minute);
		
		return cal.getTimeInMillis();
	}

 二，取当月1号

        public String getFirstDayOfMonth() {
		GregorianCalendar gCalendar = new GregorianCalendar();
		gCalendar.set(Calendar.DAY_OF_MONTH, 1);
		return dateToString(gCalendar.getTime(), "yyyy-MM-dd");
	}

 三，取上月1号

        public String getFirstDayOfLastMonth() {
		GregorianCalendar gCalendar = new GregorianCalendar();
		gCalendar.set(Calendar.DAY_OF_MONTH, 1);
		gCalendar.add(Calendar.MONTH, -1);
		return dateToString(gCalendar.getTime(),"yyyy-MM-dd");
	}