> How do I return the date for the last day of this month (September in
> this example)?

Looking for an alternate way: could this be a step in the right
direction?
(See http://chronic.rubyforge.org/)

irb(main):001:0> require 'chronic'
irb(main):011:0> Chronic.parse('last day of this month')
=> nil
Well... this was close.
:-)