Daily Average from large data set- pivot tables not applicable???

Hi,
I'm new to this forum and have looked at the data base of past threads and have not discovered a solution to my problem.
I've attached an example from a large Excel datafile of date/time stamped air pollution data recorded at 10 min intervals, there are some blanks and spaces in the data. I would like to aggregate this into daily data. A Pivot table would seem the obvious method (select days for the timestamp and average for the data) but unfortunately the last recorded value for each day (say 1/01/2005) is the value logged at Midnight and recorded as (2/01/2005 0:00). If I use a Pivot Table this value appears in the the Grouping for the 2/01/2005.

Does anybody have a robust solution that I can implement to do the data aggregation where the midnight values are included in the correct day?

cheers

Tony