public static Long getMonth1stDay() {
		cal.setTimeInMillis(System.currentTimeMillis());
		cal.set(Calendar.DATE, 1);
		return cal.getTimeInMillis();
	}
public static Long getMonth1stDay ( ) { cal . setTimeInMillis ( System . currentTimeMillis ( ) ) ; cal . set ( Calendar . DATE , 1 ) ; return cal . getTimeInMillis ( ) ; }
获取本月的第一天的时间戳
@return