时间比较的问题

b.bind_at
=> Tue, 27 Oct 2009 15:00:33 GMT +00:00
Time.now > b.bind_at + 7.days
=> false
[color=red]>> Time.now -7.days > b.bind_at[/color]
ArgumentError: comparison of Fixnum with ActiveSupport::TimeWithZone failed
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/time_with_zone.rb:302:in >'
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:insend'
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:in method_missing'
from (irb):14
[color=red]>> (Time.now -7.days) > b.bind_at[/color]
ArgumentError: wrong number of arguments (1 for 0)
from (irb):11:ininitialize'
from (irb):11:in `now'
from (irb):11
Time.now < b.bind_at + 7.days
=> true

这是什么情况？
为什么两处红色的代码会出错，谢谢解惑！
[b]问题补充：[/b]
[color=red]2.2.2[/color]
[b]问题补充：[/b]
mail ~/rails/my: ruby script/console
Loading development environment (Rails 2.2.2)

b=Blog.first.bind_at
=> Tue, 27 Oct 2009 15:00:33 GMT +00:00
Time.now > b
=> true
Time.now -7.days > b
ArgumentError: comparison of Fixnum with [color=red]ActiveSupport::TimeWithZone failed[/color]
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/time_with_zone.rb:302:in >'
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:insend'
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:in method_missing'
from (irb):4
(Time.now -7.days) > b
ArgumentError: wrong number of arguments (1 for 0)
from (irb):5:ininitialize'
from (irb):5:in `now'
from (irb):5

[b]问题补充：[/b]
[color=red]>> b=Blog.first.bind_at.utc
=> Tue Oct 27 15:00:33 UTC 2009

Time.now -7.days > b
ArgumentError: comparison of Fixnum with Time failed
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:in >'
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:insend'
from /usr/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/duration.rb:93:in method_missing'
from (irb):15[/color]
(Time.now -7.days) > b
ArgumentError: wrong number of arguments (1 for 0)
from (irb):16:ininitialize'
from (irb):16:in `now'
from (irb):16

mail ~/rails/my: ruby -v
ruby 1.8.6 (2007-09-24 patchlevel 111) [x86_64-linux]