code: public long between(DateUnit unit) {
  		long diff = end.getTime() - begin.getTime();
  		return diff / unit.getMillis();
  	}
docs: 判断两个日期相差的时长<br>
  返回 给定单位的时长差
  
  @param unit 相差的单位：相差 天{@link DateUnit#DAY}、小时{@link DateUnit#HOUR} 等
  @return 时长差