$(function() {
      $('#post_message').keyup(function() {
        var content_length = $(this).val().length;
        var remaining = 420 - content_length
        $('.counter').html(remaining);
        if (remaining < 0) {
          $('input[type=submit]').attr("disabled", "disabled");
        } else if (remaining < 21 && remaining > 9) {
          $('input[type=submit]').removeAttr("disabled");
          $('.counter').removeClass('red');
          $('.counter').addClass('dark_red');
        } else if (remaining <= 9) {
          $('input[type=submit]').removeAttr("disabled");
          $('.counter').removeClass('dark_red');
          $('.counter').addClass('red');
        } else {
          $('input[type=submit]').removeAttr("disabled");
          $('.counter').removeClass('dark_red').removeClass('red');
        }
      })