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jQuery(function(){
  $box = jQuery('.add-to-cart .qty');
	resetValue();

	jQuery($box).blur(function(){
		if(!jQuery(this).val() || (jQuery(this).val() <=0) || !jQuery.isNumeric(jQuery(this).val())) {
			jQuery(this).val(1);
		}
	});

	jQuery('.add-to-cart .btn-increment').click(function(){
		$value++;
		updateQtyBox();
	});

	jQuery('.add-to-cart .btn-decrement').click(function(){
		if($value >= 2) {
			$value--;
		}
		updateQtyBox();
	});

	function resetValue()
	{
		$value = Number(jQuery($box).val());
	}

	function updateQtyBox()
	{
		jQuery($box).val($value);
	}
});

<div class="qty-box">
  <label for="qty"><sup>*</sup> <?php echo $this->__('Qty') ?></label>
  <div class="qty-wrapper">
    <input type="text" name="qty" id="qty" maxlength="3" value="<?php echo $this->getProductDefaultQty() * 1 ?>" title="<?php echo $this->__('Qty') ?>" class="input-text qty" />
    <div class="actions">
      <a class="btn-increment" href="javascript:void(0);" title="<?php echo $this->__('Increment Value')?>"><?php echo $this->__('Increment Value')?></a>
      <a class="btn-decrement" href="javascript:void(0);" title="<?php echo $this->__('Decrement Value')?>"><?php echo $this->__('Decrement Value')?></a>
    </div>
	</div>
</div>