Given a date string, find 1 year out from that date, rounded up or down to the month. Based on a membership renewal cycle requirement.

date_test.php

<?php
function get_date($input) {
  $dateObject = new DateTime($input);
  if($dateObject->format('d') > 15) $dateObject->modify('+1 month');
  $dateObject->modify('first day of'); // set to start of month
  $Start_Date__c = $dateObject->format('Y-m-d');

  $dateObject->modify('+1 year');
  $dateObject->modify('-1 day');
  $End_Date__c = $dateObject->format('Y-m-d');

  return array($Start_Date__c, $End_Date__c);
}

$date_test = array(
  '2019-01-01' => array('2019-01-01','2019-12-31'),
  '2019-01-14' => array('2019-01-01','2019-12-31'),
  '2019-01-15' => array('2019-01-01','2019-12-31'),
  '2019-01-16' => array('2019-02-01','2020-01-31'),
  '2019-06-16' => array('2019-07-01','2020-06-30'),
  '2019-10-05' => array('2019-10-01','2020-09-30'),
  '2019-10-17' => array('2019-11-01','2020-10-31'),
  '2019-11-10' => array('2019-11-01','2020-10-31'),
  '2019-11-20' => array('2019-12-01','2020-11-30'),
  '2019-12-08' => array('2019-12-01','2020-11-30'),
  '2019-12-16' => array('2020-01-01','2020-12-31'),
);

foreach($date_test as $input => $output) {
  $result = get_date($input);
  if($result[0] != $output[0] || $result[1] != $output[1]) {
     echo 'Failed for ' . $input . PHP_EOL;
     echo '  expected ' . print_r($output, true) . ' received ' . print_r($result, true) . PHP_EOL;
     die();
  }
  else {
    echo 'Passed for ' . $input . PHP_EOL;
  }
}