With start datetime in cell C1 and enddate time in cell D1 try the below
array formula. Press CTRL+SHIFT+ENTER to enter the formula. If successful in
'Formula Bar' you can notice the curly braces at both ends like "{=formula}"

=AVERAGE(IF((A1:A10=C1)*(A1:A10=D1),B1:B10))

--
Jacob (MVP - Excel)


"Chris26" wrote:

Hi
I have the following data
Col-A Col-B
1/10/2009 10:20 0.006
1/10/2009 10:22 0.007
1/10/2009 10:24 0.006
.......
1/06/2010 15:00 0.015

etc
What I would like to be able to do, is to calculate the average of the
vaules in Col B only between certain time periods i.e. 2am and 4am for each
day in the data set

Any help appreciated.
Many Thanks
Chris