Error when using function in template

I get the following error:

Fatal error: Using $this when not in object context in /home/ssec/public_html/wp-content/plugins/last-modified-timestamp/last-modified-timestamp.php on line 103

…when doing this in my theme template file:

<?php
if( function_exists(‘get_the_last_modified_timestamp’) ) :
$modifiedDate = get_the_last_modified_timestamp();
?>
<meta name=”date” content=”<?php echo $modifiedDate; ?>” scheme=”” />
<?php endif; ?>

Thoughts?

https://wordpress.org/plugins/last-modified-timestamp/