In regular JRuby irb:

jruby-1.6.4 :004 > require 'time'
 => false
jruby-1.6.4 :005 > Time.now.utc.iso8601
 => "2011-12-16T07:33:42Z"
jruby-1.6.4 :006 >


In the debugger :

(rdb:1) set autoeval on
autoeval is on.
(rdb:1) require 'time'
false
(rdb:1) Time.now
2011-12-16 09:47:46 -0500
(rdb:1) Time.now.utc
2011-12-16 14:47:47 UTC
(rdb:1) Time.now.utc.iso8601
NoMethodError Exception: undefined method `iso8601' for 2011-12-16 14:47:49 UTC:Time
(rdb:1)