You seem to have a misapprehension about what code is being run where: :onchange is a javascript fragment - you need to write a chunk of javascript to do this. that chunk of javascript cannot do anything to any ruby variables (other than by causing a request to be made). The reason you get the error that you get is that ruby evaluates start_date_time = $F, end_date = end_date_time[1,4] at the point that your template is rendered and you presumably don't use a global variable called $F.
I've never used calendardateselect and I'm not entirely sure what you're trying to do, but either you need to do some client side stuff or you need something in your action to preprocess what gets submitted.
Fred