class ActionView::Helpers::DateTimeSelector

def css_class_attribute(type, html_options_class, options) # :nodoc:

:nodoc:
=> "date optional my-year"
css_class_attribute(:year, 'date optional', { year: 'my-year' })

Builds the CSS class value for the select element. 
def css_class_attribute(type, html_options_class, options) # :nodoc:
  css_class = \
    case options
    when Hash
      options[type.to_sym]
    else
      type
    end
  [html_options_class, css_class].compact.join(" ")
end