If you subtract two Date or DateTime objects, the result is a Rational representing the number of days between them. What you need is:
a = Date.new(2009, 10, 13) - Date.new(2009, 10, 11)
(a * 24 * 60).to_i # 2880 minutes
or
a = DateTime.new(2009, 10, 13, 12, 0, 0) - DateTime.new(2009, 10, 11, 0, 0, 0)
(a * 24 * 60).to_i # 3600 minutes