class ActionView::Helpers::DateTimeSelector

def month_names

name array has a leading +nil+ element.
Returns translated month names, but also ensures that a custom month 
def month_names
  @month_names ||= begin
    month_names = @options[:use_month_names] || translated_month_names
    month_names = [nil, *month_names] if month_names.size < 13
    month_names
  end
end