Subtract this year's value from value at same time last year on report

[Susy]

I have a report where I need to calculate an annual
difference ([depth] lastmonth/lastyear - [depth]
thismonth/thisyear). I also have to calculate a [depth]
minimum winter value minus a [depth] maximum summer
value. The depth measurements are sometimes sporadic, so
not every month is measured.

Can I do this in controls on the report or do I have to do
a query?

Thanks.