Ok so heres my problem.
I have a workbook which has the following columns:

Date, A/C Type, Rego, PIC, Co-Pilot, and then 13 columns of different types
of flying.

What I need, and have been trying to figure out, is a formula which will
compare todays date to that of 30 and 90 days ago and then add all the data
within the 13 columns to give me the total number of hours flown in the
previous 30 and 90 days, on any aircraft type, configuration, etc.

I have tried SUMIF, COUNTIF, and a few other basic formulas, but for some
reason any time I try and add in TODAY, NOW, etc it makes the formula
volitile and either returns an error or 0.

If anyone can give me any ideas on which formulas to try, it would be a
great help!