Time picker help

I have installed time picker in my application using the below url.

ruby script/plugin install http://timepicker.rubyforge.org/svn/trunk/

<%=time_picker(Time.now, {:time_format => ‘12’, :minute_step => 30})%>

when i use like above in my list.rhtml file , i get error like below

undefined method `leading_zero_on_single_digits’ for

how i should use it…

kindly help me out to get it working

Thanks for any help

Is that not possible?