difference between two dates in SQL

I thought I was doing this right, but I am getting weird results so I
thought I would quickly ask the group.

I am passing in today's date as a var (date data type) which I am using to
compare against a date field.

My sql statement is as follows:

select * from myfile where (:wtoday - ssodat) > 30 and (:wtoday - ssodat)
< 61;

I thought that substracting two dates from each other would return a
result in days is this not correct?

If not, any suggestions on how to accomplish this?

Douglas