无法识别Jekyll插件

我正试图复制Jekyll插件示例 https://github.com/mojombo/jekyll/wiki/Plugins 要呈现*render_time*标记,但出现以下错误:

/Library/Ruby/Gems/1.8/gems/liquid-2.0.0/lib/liquid/block.rb:58:in `unknown_tag': Unknown tag 'render_time' (Liquid::SyntaxError)
from /Library/Ruby/Gems/1.8/gems/liquid-2.0.0/lib/liquid/block.rb:28:in `parse'
from /Library/Ruby/Gems/1.8/gems/liquid-2.0.0/lib/liquid/document.rb:5:in `initialize'
from /Library/Ruby/Gems/1.8/gems/liquid-2.0.0/lib/liquid/template.rb:58:in `new'
from /Library/Ruby/Gems/1.8/gems/liquid-2.0.0/lib/liquid/template.rb:58:in `parse'
from /Library/Ruby/Gems/1.8/gems/liquid-2.0.0/lib/liquid/template.rb:46:in `parse'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/../lib/jekyll/convertible.rb:72:in `do_layout'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/../lib/jekyll/page.rb:79:in `render'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/../lib/jekyll/site.rb:158:in `render'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/../lib/jekyll/site.rb:154:in `each'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/../lib/jekyll/site.rb:154:in `render'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/../lib/jekyll/site.rb:99:in `process'
from /Library/Ruby/Gems/1.8/gems/jekyll-0.5.7/bin/jekyll:129
from /usr/bin/jekyll:19:in `load'
from /usr/bin/jekyll:19

我的插件在*/_plugins/homepage.rb*中,_config.yml包含标准规则:

source:      .
destination: ./_site
plugins:     ./_plugins

有人有同样的问题吗?