Average date from 1 minute readings to 30 minute readings

I have data taken on 1 minute intervals. I need to average it into 30 minute averages. I am currently doing it manually. Please help!
Example below:

05/01/2010 12:30:00 AM 5/1/10 12:30:00 AM 12.6 12.59666667
05/01/2010 12:31:00 AM 5/1/10 12:31:00 AM 12.6
05/01/2010 12:32:00 AM 5/1/10 12:32:00 AM 12.6
05/01/2010 12:33:00 AM 5/1/10 12:33:00 AM 12.6
05/01/2010 12:34:00 AM 5/1/10 12:34:00 AM 12.6
05/01/2010 12:35:00 AM 5/1/10 12:35:00 AM 12.7
05/01/2010 12:36:00 AM 5/1/10 12:36:00 AM 12.7
05/01/2010 12:37:00 AM 5/1/10 12:37:00 AM 12.6
05/01/2010 12:38:00 AM 5/1/10 12:38:00 AM 12.6
05/01/2010 12:39:00 AM 5/1/10 12:39:00 AM 12.7
05/01/2010 12:40:00 AM 5/1/10 12:40:00 AM 12.6
05/01/2010 12:41:00 AM 5/1/10 12:41:00 AM 12.6
05/01/2010 12:42:00 AM 5/1/10 12:42:00 AM 12.6
05/01/2010 12:43:00 AM 5/1/10 12:43:00 AM 12.6
05/01/2010 12:44:00 AM 5/1/10 12:44:00 AM 12.6
05/01/2010 12:45:00 AM 5/1/10 12:45:00 AM 12.6
05/01/2010 12:46:00 AM 5/1/10 12:46:00 AM 12.6
05/01/2010 12:47:00 AM 5/1/10 12:47:00 AM 12.6
05/01/2010 12:48:00 AM 5/1/10 12:48:00 AM 12.6
05/01/2010 12:49:00 AM 5/1/10 12:49:00 AM 12.6
05/01/2010 12:50:00 AM 5/1/10 12:50:00 AM 12.6
05/01/2010 12:51:00 AM 5/1/10 12:51:00 AM 12.6
05/01/2010 12:52:00 AM 5/1/10 12:52:00 AM 12.6
05/01/2010 12:53:00 AM 5/1/10 12:53:00 AM 12.6
05/01/2010 12:54:00 AM 5/1/10 12:54:00 AM 12.6
05/01/2010 12:55:00 AM 5/1/10 12:55:00 AM 12.6
05/01/2010 12:56:00 AM 5/1/10 12:56:00 AM 12.5
05/01/2010 12:57:00 AM 5/1/10 12:57:00 AM 12.5
05/01/2010 12:58:00 AM 5/1/10 12:58:00 AM 12.5
05/01/2010 12:59:00 AM 5/1/10 12:59:00 AM 12.5

I was hoping for a formula since the data is set on a weekly basis with 12,000 +/- readings and it takes a while to drag. I also have blank rows for day seperators.