<?php

echo Date("Y-m-d", strtotime("2013-01-01 +1 Month -1 Day")) . "\n";
  // 2013-01-31

echo Date("Y-m-d", strtotime("2013-01-31 +1 Month -3 Day")) . "\n";
  // 2013-02-28

echo Date("Y-m-d", strtotime("2013-01-31 +2 Month")) . "\n";
  // 2013-03-31

echo Date("Y-m-d", strtotime("2013-01-31 +3 Month -1 Day")) . "\n";
  // 2013-04-30

echo Date("Y-m-d", strtotime("2013-12-31 -1 Month -1 Day")) . "\n";
  // 2013-11-30

echo Date("Y-m-d", strtotime("2013-12-31 -2 Month")) . "\n";
  // 2013-10-31

echo Date("Y-m-d", strtotime("2013-12-31 -3 Month")) . "\n";
  // 2013-10-01

echo Date("Y-m-d", strtotime("2013-12-31 -3 Month -1 Day")) . "\n";
  // 2013-09-30
    

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