I have hourly data from an instrument and would like to reduce the data set
(see example below) to one point per day, either by selecting a specific time
(e.g., 00:00) each day, or by averaging all of the values for one day. The
data files may contain more than one month's worth of data per file (i.e.,
quite large files).


8/11/2006 00:00 17.06
8/11/2006 01:00 17.05
8/11/2006 02:00 17.05
8/11/2006 03:00 17.05
8/11/2006 04:00 17.05
8/11/2006 05:00 17.05
8/11/2006 06:00 17.06
8/11/2006 07:00 17.06
8/11/2006 08:00 17.05
8/11/2006 09:00 17.06
8/11/2006 10:00 17.06
8/11/2006 11:00 17.06
8/11/2006 12:00 17.06
8/11/2006 13:00 17.06
8/11/2006 14:00 17.06
8/11/2006 15:00 17.06
8/11/2006 16:00 17.06
8/11/2006 17:00 17.06
8/11/2006 18:00 17.06
8/11/2006 19:00 17.05
8/11/2006 20:00 17.06
8/11/2006 21:00 17.06
8/11/2006 22:00 17.06
8/11/2006 23:00 17.06
8/12/2006 00:00 17.06
8/12/2006 01:00 17.05
8/12/2006 02:00 17.06
8/12/2006 03:00 17.06
8/12/2006 04:00 17.06
8/12/2006 05:00 17.06
8/12/2006 06:00 17.06
8/12/2006 07:00 17.06
8/12/2006 08:00 17.06