Header: Include Date Published and Date Modified

You can check if is_singular page/post and then use global $post to get the post ID outside the loop.
Also for the modified date you should use get_the_modified_date instead because you’re already echoing.

<?php if(is_singular()): ?>
    <?php global $post; ?>
    <meta name="date" content="<?php echo get_the_date( 'm-d-Y', $post->ID ); ?>">
    <meta name="revised" content="<?php echo get_the_modified_date( 'm-d-Y', $post->ID ); ?>">
 <?php endif; ?>