style / different content posts based on month

I’d like to style a post differently based on whether its this month, or any other previous month.

I.e. something like…

if (this_month) { get_template_part( 'content', 'post1' ) }
else if (any_other prior_month) { get_template_part( 'content', 'post2') }

How can I go about this?

I found this snippet which is showing how to do a conditional statement based on time but don’t quite understand it:

<?php $w_h = $w_d = 0; ?>
<?php while (have_posts()) : the_post(); ?>
<?php
    if ( date('Yz') == get_the_time('Yz') ) {
        if (!$w_d++) echo 'Today<br />';
    } elseif ( date('Yz')-1 == get_the_time('Yz') ) {
        if (!$w_h++) echo 'Yesterday<br />';
    } else {
        echo the_date();
    };
?>