try to display the products_last_modified in tpl_product_info_display.php not work

I just try to display the products_last_modified in tpl_product_info_display.php, but if I add the line, it does not display anything.

  <?php echo ( '<li>' . "last Modified time" . $products_last_modified . '</li>' : '') . "\n"; ?>

<!--bof Product details list  -->
<?php if ( (($flag_show_product_info_model == 1 and $products_model != '') or ($flag_show_product_info_weight == 1 and $products_weight !=0) or ($flag_show_product_info_quantity == 1) or ($flag_show_product_info_manufacturer == 1 and !empty($manufacturers_name))) ) { ?>
<ul id="productDetailsList" class="floatingBox back">
  <?php echo (($flag_show_product_info_model == 1 and $products_model !='') ? '<li>' . TEXT_PRODUCT_MODEL . $products_model . '</li>' : '') . "\n"; ?>
  <?php echo (($flag_show_product_info_weight == 1 and $products_weight !=0) ? '<li>' . TEXT_PRODUCT_WEIGHT .  $products_weight . TEXT_PRODUCT_WEIGHT_UNIT . '</li>'  : '') . "\n"; ?>
  <?php echo (($flag_show_product_info_quantity == 1) ? '<li>' . TEXT_PRODUCT_QUANTITY . $products_quantity . '</li>'  : '') . "\n"; ?>
  <?php echo (($flag_show_product_info_manufacturer == 1 and !empty($manufacturers_name)) ? '<li>' . TEXT_PRODUCT_MANUFACTURER . $manufacturers_name . '</li>' : '') . "\n"; ?>
  <?php echo ( '<li>' . "last Modified time" . $products_last_modified . '</li>' : '') . "\n"; ?>
</ul>
<br class="clearBoth" />

I am using zen cart 1.3.8