Requirements:

I'm creating a user request form. Users need to be able to enter the start
and end dates of requests. I also need there to be three separate fields
that will automatically have the average hours of sunlight per day in the
month requested and average number of overcast days, depending on which month
the request is for and the total day length of the request.

Problems:

I can't figure out how to get the total number of days calculated AND the
average day length and the average number of overcast days entered
automatically. If I have a drop-down list for the dates that users will
select, I can figure out how to get the averages filled in automatically
using VLOOKUP, but I don't know how I can get it to calculate the number of
days in the request.

Example of what I have as of right now:

(Requested Start Dates)
Cell A1 = Drop down list - Jan thru Dec
Cell B1 = Drop down list - 1 thru 31
Cell C1 = Drop down list - 2005 thru 2010

(Requested End Dates)
Cell A2 = Drop down list - Jan thru Dec
Cell B2 = Drop down list - 1 thru 31
Cell C2 = Drop down list - 2005 thru 2010

What I need in other fields:

Cell D1 = Should calculate total number of days in request
Cell D2 = Should automatically fill in the average amount of sunlight per
day in the request, depending on the month selected
Cell D3 = Should automatically fill in the average number of overcast days
in the month requested

I hope this isn't confusing.

Thanks!