Relative Time
"first day *of* next month"
modify( 'first day of next month' )->format( 'Y-m-d' ), "\n";
?>]]>
Explanation:
2018-01-31 15:48:21
2018-|ef2929|02|-31 15:48:21
"next month" increases month number by 1
2018-02-|ef2929|01| 15:48:21
"first day of" resets the day number to 1