Ruby on Rails

i am try to change existing code,i am not unserstanding code,it is
written for blog archive clicking on each month and year in right column
it should redirect to that clicked month.by the below code only few are
working not all months,december is giving invalid date error
div class="right-col">
<h2>ARCHIVES</h2>
<div class='posts_by_month'>

<ul>
<% archive_string = "" %>
<% date_value = "" %>
<% @posts_by_month.each do |monthname, posts| %>

<% date_value = (Date::MONTHNAMES[monthname[0..-6].to_i]) + "
" + (monthname[3..8]) %>
<% archive_string = "<li><a class='filter' href='/blog?month="
+ monthname[0..-6] + "&year=" + monthname[3..8] + "'>" + date_value +
"</a></li>" + archive_string %>
<% end %>
<%= raw archive_string %>
</ul>

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