Howto Overwrite existing Ruby Class via Rails app?

Look for Rails i18n helpers, they do what you're looking for.

No that's not the problem, in fact I'am using I18n. But that only
works for translating /localizing your data from the database.
But now I'm trying to Post data to the database. So for instance, if I
would have a form with a text field: date_attended (<%=
f.text_field :date_attended%> and would post the following 'SPANISH'
date to the database: 10 Abril 2009 (-> which is April 10, 2009 in
English) The database won't understand this.

So therefore I need to change this file: 1.8/usr/lib/ruby/1.8/date/
format.rb, because when I do that it works, but I want to do it via my
rails app. I want to overwrite it there, but have no idea how. I was
also thinking of maybe translating the months in the model itself, so
that the database understands which month I'm talking about. But also
I have no idea how to implement this in my code. I need (code)samples/
guidance, any suggestion is welcome.