New shortcode [seomonth] that renders next month from 21st of this month

seomonth.php

<?php
// you can change 20 on line #8 to any date between 1 to 27
add_shortcode( 'seomonth' , 'gtrmd_seomonthyear' );
    function gtrmd_seomonthyear() {
    $gtseomonthyear1 = date_i18n("F");
    $gtseomonthyear2 = date("F", strtotime ( '+1 month' , strtotime ( $gtseomonthyear1 ) )) ;
    $gtdtseocheck = date("j");
    if($gtdtseocheck > 20){
        return "$gtseomonthyear2"; 
        } else{
        return "$gtseomonthyear1";
    }
}
?>