Although result of Fixnum#day, Fixnum#days, Fixnum#week, etc looks
like a new Fixnum with seconds in given period it's actually not a Fixnum.
Let's take a quick look that might lead us toward pretty wrong direction:
1.week.is_a? Fixnum # => true
1.week.instance_of? Fixnum # => true
1.week.class # => Fixnum