<?php
$year = 2012;
$month = 1;
$inputDatePast = intval(sprintf('%4d%02d', $year, $month)); //201201

$year = 2016;  // if you read this after september 2016: add a few more years here ;)
$month = 9;
$inputDateFuture = intval(sprintf('%4d%02d', $year, $month)); //201609

$prevMonth = intval(date('Ym', mktime(12, 0, 0, date('n')-1, 1, date('Y'))));

$validator = new Zend_Validate_GreaterThan($prevMonth);
$validator->isValid($inputDatePast); // false
$validator->isValid($inputDateFuture); // true
?>