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