Hi,
I'm running the latest one-click-installer (1.8.2-14) on a WinXP box.
With
require 'Date'
you'll likely get an error, but with
require 'date'
you won't...
[irb session begin] ------------------------------------
E:\>irb
irb(main):001:0> require 'Date'
c:/ruby/lib/ruby/1.8/Date.rb:238: warning: already initialized constant MONTHNAMES
c:/ruby/lib/ruby/1.8/Date.rb:243: warning: already initialized constant DAYNAMES
c:/ruby/lib/ruby/1.8/Date.rb:247: warning: already initialized constant ABBR_MONTHNAMES
c:/ruby/lib/ruby/1.8/Date.rb:250: warning: already initialized constant ABBR_DAYNAMES
c:/ruby/lib/ruby/1.8/Date.rb:254: warning: already initialized constant ITALY
c:/ruby/lib/ruby/1.8/Date.rb:258: warning: already initialized constant ENGLAND
c:/ruby/lib/ruby/1.8/Date.rb:262: warning: already initialized constant JULIAN
c:/ruby/lib/ruby/1.8/Date.rb:266: warning: already initialized constant GREGORIAN
NameError: undefined method `today' for `DateTime'
from c:/ruby/lib/ruby/1.8/Date.rb:1261:in `undef_method'
from c:/ruby/lib/ruby/1.8/Date.rb:1261
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/loadpath_manager.rb:5:in `require__'
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/loadpath_manager.rb:5:in `require'
from (irb):1
[irb session end] ------------------------------------
This doesn't seem to be an irb issue since running "require 'date'" outside irb yields the same result:
E:\>ruby -v
ruby 1.8.2 (2004-12-25) [i386-mswin32]
E:\>ruby -r Date -e ''
c:/ruby/lib/ruby/1.8/date.rb:238: warning: already initialized constant MONTHNAMES
---------------- [snipped part of the output] ----------------
BTW, on the identical hardware running Linux I get...
[stk@tao ~]$ ruby -v
ruby 1.8.2 (2004-12-25) [i686-linux]
[stk@tao ~]$ ruby -r date -e ''
[stk@tao ~]$
... no error, while 'Date' raises a file not found Error.
Aha, I think this happens: On Linux everything works fine. 
On Windows however, the case dependency with respect to filenames
is sub-optimal. Requiring the file with the 'correct' file name
'date' yields the expected behaviour ('require' returns false).
But requiring a slightly misspelled filename as 'Date' leads to
the following:
1. 'Date' is not found in the list of already loaded files.
2. Loading the file doesn't yield an Error - because Windows
finds 'date'.
3. Bad things happen on a Windows box, while processing that file.
Happy rubying
Stephan