You have to remove the 24 hour time from the Date/time format. Dates in
excel start at midnight Jan 1, 1900 = 1. Every day adds one to the time an
hour equals 1/24. You need to remove the fraction part of the time from the
whole numbers.

3/30/09 at 12:00 PM = 39902.5

The .5 is from noon being 1/2 of a day. You want the hour which is the .5

Use this equation
=mod(A1,1)

where A1 is the date/time you are converting to hourly time.

"Lindsey" wrote:

I am working with a dataset that contains 3 years of hourly temperature data
that I am trying to graph in order to QA/QC. How can I tell Excel to average
the hourly data into daily data, or put it in a language so it can be graphed
as a time-series?