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<?php printf("2013-10-31, -1 month: %s\n2013-10-31, first day of 1 month ago: %s", date('m', strtotime('2013-10-31 -1 month')), date('m', strtotime('2013-10-31 first day 1 month ago')));
Output for 4.3.0 - 7.1.0
2013-10-31, -1 month: 10 2013-10-31, first day of 1 month ago: 09