新手求一个问题

各位大侠，我这有一组数据，格式如下 :
        A            B  
12/16/11 07:10:03  824-S1
12/16/11 07:10:03  824-S2
12/16/11 07:10:03  824-S3
12/16/11 07:14:03  824-S1
12/16/11 07:14:03  824-S3
12/16/11 07:14:03  824-S2
12/16/11 07:18:03  824-S1
12/16/11 07:18:03  824-S3
12/16/11 07:18:03  824-S2
12/16/11 07:22:03  824-S1
12/16/11 07:22:03  824-S2
12/16/11 07:22:03  824-S3
12/16/11 09:06:03  824-S3
12/16/11 09:06:03  824-S2
12/16/11 09:06:03  824-S1
12/16/11 09:10:03  824-S1
12/16/11 09:10:03  824-S3
12/16/11 09:10:03  824-S2
12/16/11 09:14:04  824-S1
12/16/11 09:14:04  824-S2
12/16/11 09:14:04  824-S3
12/16/11 09:18:03  824-S2
12/16/11 09:18:03  824-S3
12/16/11 09:18:03  824-S1
12/16/11 09:22:04  824-S1
12/16/11 09:22:04  824-S2
12/16/11 09:22:04  824-S3
12/16/11 09:26:03  824-S3
12/16/11 09:26:03  824-S2
12/16/11 09:26:03  824-S1
12/16/11 09:30:05  824-S1
12/16/11 09:30:05  824-S2
12/16/11 09:30:05  824-S3
12/16/11 09:34:04  824-S3
12/16/11 09:34:04  824-S2
12/16/11 09:34:04  824-S1
12/16/11 09:38:04  824-S1
12/16/11 09:38:04  824-S3
12/16/11 09:38:04  824-S2
12/16/11 09:42:03  824-S1
12/16/11 09:42:03  824-S2
12/16/11 09:42:03  824-S3
12/16/11 09:46:04  824-S2
12/16/11 09:46:04  824-S1
12/16/11 09:46:04  824-S3
12/16/11 09:50:04  824-S1
12/16/11 09:50:04  824-S2
12/16/11 09:50:04  824-S3
12/16/11 09:54:03  824-S3
12/16/11 09:54:03  824-S1
12/16/11 09:54:03  824-S2
12/16/11 09:58:03  824-S1
12/16/11 09:58:03  824-S3
12/16/11 09:58:03  824-S2
12/16/11 10:02:04  824-S1
12/16/11 10:02:04  824-S3
12/16/11 10:02:04  824-S2
12/16/11 10:06:04  824-S2
12/16/11 10:06:04  824-S1
12/16/11 10:06:04  824-S3
12/16/11 10:10:04  824-S1
12/16/11 10:10:04  824-S2
12/16/11 10:10:04  824-S3A列是时间，B列是值。我现在想做一个筛选，就是如果B列中存在相同的值，同时如果B列值对应的时间连续间隔是3：59秒,4:00秒或4:01秒,那我只保留时间最早的那一个;如果连续间隔超过4:01秒,那我也要保留下一个....举个例子:
就这组据来看,比如前面一段:
12/16/11 07:10:03  824-S1
12/16/11 07:10:03  824-S2
12/16/11 07:10:03  824-S3
12/16/11 07:14:03  824-S1
12/16/11 07:14:03  824-S3
12/16/11 07:14:03  824-S2
12/16/11 07:18:03  824-S1
12/16/11 07:18:03  824-S3
12/16/11 07:18:03  824-S2
12/16/11 07:22:03  824-S1
12/16/11 07:22:03  824-S2
12/16/11 07:22:03  824-S312/16/11 07:10:03  824-S1,  12/16/11 07:14:03  824-S1,  12/16/11 07:18:02  824-S1,
12/16/11 07:22:04  824-S1这几条记录的值都是824-S1,但是时间间隔是3:59秒,4:00,4:01秒,所以我只要保留时间最早的那一条,也就是12/16/11 07:10:03  824-S1条.同样,对于824-S2,824-S3也是取最早的那一条.但是对于下面的
12/16/11 07:22:03  824-S1
12/16/11 07:22:03  824-S2
12/16/11 07:22:03  824-S3
12/16/11 09:06:03  824-S3
12/16/11 09:06:03  824-S2
12/16/11 09:06:03  824-S1
12/16/11 09:10:03  824-S1
12/16/11 09:10:03  824-S3
12/16/11 09:10:03  824-S2
12/16/11 09:14:04  824-S1
12/16/11 09:14:04  824-S2
12/16/11 09:14:04  824-S3里面有这样的数据:
12/16/11 07:22:03  824-S3
12/16/11 09:06:03  824-S3
虽然824-S3一样,但是由于时间间隔不是3:59,4:00,4:01秒,所以我还是需要保留12/16/11 09:06:03  824-S3,那下面的12/16/11 09:06:03  824-S3,  12/16/11 09:10:03  824-S3, 12/16/11 09:14:04  824-S3这三条由于间隔又是4分钟,我就不需要了....不知道上面的叙述清楚没有啊,呵呵