time based expiration for fragment caching

Does anybody have any suggestions on how to do this? I have tried all
kinds of plugins , googled all over the place but can't get it to work
for 2-3 days now:

I use simple fragement caching like this:
<% cache do %>
<%= render :partial => @groups %>
<% end %>

and in the controller somthing like this:
  def index
   unless read_fragment 'index'
    fetch_groups nil
    end
  end

THIS DOES WORK. I am able to expire this cache from the controller. I
have not changed any of the default caching settings.

Now I am looking for a solution to achieve automatic time based
expiration of such a fragment cache.(I didn't get plugins like
cache_foo, timed_fragment_cached to work.) I think it might have to do
with memcached issue. As a lrealtive newbie I am confused whether
memcached is a.) needed for this b.) already installed by default or
not.

Can anybody point me to a relatively easy way on how to do this?

Thanks so much!
Justus

Ok, let me rephrase this:

Is there a simple way to do time based expiration for cached fragements
WITHOUT using memcached?

Thanks,
Justus