.click() or .animate(); not working in Webkit or IE (works fine in Firefox)

.click() or .animate(); not working in Webkit or IE (works fine in Firefox)

This works fine in Firefox but not in the Webkit browsers or IE

$(document).ready(function(){
  $('#lastweek').hide();
  $('#nextweek').click(function(){
      var lvalue = parseInt($('#weekholder').css('left'));
      if(lvalue > '-818'){
      $('#weekholder').animate({left: '-=273'}, 'slow');
      $('#lastweek').show();
    }
    else{
        $('#nextweek').hide();
    } 
     
  });
 
  $('#lastweek').click(function(){
      var lvalue = parseInt($('#weekholder').css('left'));
      if(lvalue < '0'){
      $('#weekholder').animate({left:'+=273'}, 'slow');
      $('#nextweek').show();
    }
    else{
        $('#lastweek').hide();
    } 
   
   
     
  });