How can I make thumbnails a display different size/position if the blog post was posted before a specific date?

In what way is it affecting your post thumbnails?

I cleaned up your code a bit. If you can show me the output you’re getting I’ll be able to help you further.

<?php
    global $post;

    $compare_date = strtotime( "2018-03-01" );
    $post_date    = strtotime( $post->post_date );

    if ( has_post_thumbnail() ) {
        if ( $compare_date < $post_date ) {
            echo '<div style="float:right;padding-left:20px;"><img src="' . the_post_thumbnail() . '"/></div>';
        }
        else {
            echo '<img src="' . the_post_thumbnail() . '" />';
        }
    }
?>