I was unable to reproduce this on master:

$ script/rails c
13.Loading development environment (Rails 3.0.0)
>> 13.days
=> 13 days
>> exit
$ script/rails c production
Loading production environment (Rails 3.0.0)
>> 13.days
=> 13 days
>> exit

I was unable to reproduce this on rails 3.0.0:

$ script/rails c
>> a = Date.new(2010,9,30)
=> Thu, 30 Sep 2010
>> a.end_of_week
=> Sun, 03 Oct 2010
>> exit
$ script/rails c production
Loading production environment (Rails 3.0.0)
>> a = Date.new(2010,9,30)
=> Thu, 30 Sep 2010
>> a.end_of_week
=> Sun, 03 Oct 2010
>> exit

Neither was I:

➜  script/rails c
Loading development environment (Rails 3.1.0.beta)
ruby-1.9.2-p0 > a = Date.new(2010, 9, 30)
 => Thu, 30 Sep 2010 
ruby-1.9.2-p0 > a.end_of_week
 => Sun, 03 Oct 2010 
ruby-1.9.2-p0 > exit
➜  script/rails c production
Loading production environment (Rails 3.1.0.beta)
ruby-1.9.2-p0 > a = Date.new(2010, 9, 30)
 => Thu, 30 Sep 2010 
ruby-1.9.2-p0 > a.end_of_week
 => Sun, 03 Oct 2010 
ruby-1.9.2-p0 > exit

(Edge Rails).

I just realize now that, as you all say, it's working properly with a new rails 3.0.0 application, my mistake.

However I'm still having this wired behaviour with an app I'm currently working on (http://github.com/Itnig/intranet3). Tested with ubuntu and mac platforms I was unable to reach the source of this problem.

Anyway, looking at the implementations of end_of_week and beginning_of_week methods in activesupport/lib/active_support/core_ext/date/calculations.rb I still believe that 'result = self + days_to_sunday.days' (line 180) should be replaced with 'result = self + days_to_sunday'.
The same change has already been made in beginning_of_week method, symetrical of end_of_week.

I am also getting this error.

Loading production environment (Rails 3.0.0)
irb(main):001:0> 13.days
=> 1123200
irb(main):002:0> a = Date.new(2010,9,30)
=> Thu, 30 Sep 2010
irb(main):003:0> a.end_of_week
=> Mon, 31 May 2720

Though this is happening in both development and production environments.
I have tested on an OSX machine and also Linux box with same results.
Im using ruby 1.8.7 (2009-06-12 patchlevel 174) on both machines.

Still can't reproduce this using the system ruby on Mac OS X. Looking at your irb output it seems as though 13.days is returning an integer instead of a duration. Is there a plugin or gem that may have overwritten the days method? The other possibility is an old version of ActiveSupport::CoreExtensions::Numeric::Time being included - in Rails 1.2.x it returns an integer and not a duration.

I will look into the possibility that a gem or plugin is overriding the days method.
The weird thing is that this only started happening tonight, no new gems or plugins we added but I did do 3 small changes to the app, Though I would not expect those changes to effect the days method nor override it.

Oh I lied!
I did add a new gem to my Gemfile, I just did it late last night and had not got around to deploying the changes.
God was the culprit, not that I can remember why I wanted to add it to my Gemfile.
It was a long one last night.

Thanks for putting me on the right track.

You were right, the 'god' gem (god-0.11.0) was causing the problem redefining .days method in /lib/god/sugar.rb:20.

Thanks for your advice, finally solved!