Add modified date to published date

Hi, I have been a WordPress user for a long time, so some of my articles are quite old, but still usefull for my readers. I update them regularly. That’s why I would like to add the last modified date to the publish date. The code to do so should be:
`$u_time = get_the_time(‘U’);
$u_modified_time = get_the_modified_time(‘U’);
if ($u_modified_time >= $u_time + 86400) {
echo “<p>Last modified on “;
the_modified_time(‘F jS, Y’);
echo ” at “;
the_modified_time();
echo “</p> “; }

But where (in which file and in which line) should I add this code? Preferably the published date with the ‘clock-icon’ should be published first and then on the same line the modified date.

Thanks,
With kind regards
Nico