Problem using rake to expire caches

I am trying to expire a fragment cache in rake. I know that the best
practice to expire the cache is in the Sweepers. But as I have to
expire
the cache at the end of the month I was thinking on the lines of doing
it in a rake task.
I had tried this but it showed me an error while running the task

expire_fragment(:controller => :login, :action => :index, :something=>
"1")

Well it gives me a "No method Error"

Wanted to know what is the best practice to clear the cache on a cron
job? Should a sweeper action be called or a rake task is prescribed?

If any one has done this would highly appreciate if he/she could
provide
the code for calling a sweeper action from a cron or can someone point
out what I am missing in the rake file. Do I have to create an object
and then call the method if so then an object of what class?

Please pardon me for being such a newbie. Any help would be highly
appreciated.

Cheers,
Jazzy