public static long between(Date beginDate, Date endDate, DateUnit unit) {
		return between(beginDate, endDate, unit, true);
	}
public static long between ( Date beginDate , Date endDate , DateUnit unit ) { return between ( beginDate , endDate , unit , true ) ; }
判断两个日期相差的时长，只保留绝对值

@param beginDate 起始日期
@param endDate 结束日期
@param unit 相差的单位：相差 天{@link DateUnit#DAY}、小时{@link DateUnit#HOUR} 等
@return 日期差