strptime2 - parses dates given a format

examples.php

<?php
require 'strptime2.php';

echo '<pre>';

echo strptime2('12/29/1989', 'm/d/Y') . "\n";
echo strptime2('7/15/1989 10:13am', 'n/d/Y h:ia') . "\n";
echo strptime2('07/15/1989 10:13AM', 'm/d/Y h:iA') . "\n";
echo strptime2('Sep 12, 2013 22:12', 'M j, Y H:i') . "\n";
echo strptime2('Thursday, July 26, 2012 - 2:48:12PM', 'l, F j, Y - g:i:sA') . "\n";
echo strptime2('February 29, 2008', 'F d, Y') . "\n"; // leap year

echo '</pre>';

strptime2.php

<?php
/**
 * A strptime() alternative.
 *
 * Large amounts of this code were inspired by the Drupal module Scheduler's
 * _scheduler_strptime() function as well as the comment left on php.net:
 * http://www.php.net/manual/en/function.strptime.php#82004
 *
 * @param string $time
 *   The string to be turned into a Unix Timestamp.
 * @param string $format
 *   The string representing a format understood by date().
 *
 * @return int
 *   The integer that represents a Unix Timestamp or FALSE if failure.
 *
 * @see
 *   date()
 */
function strptime2($time, $format) {
  // This associative array maps characters that the date() function understands
  // with their regex validator.
  $characters = array(
    '/' => '\/',
    'd' => '(\d{2})',
    'D' => '(\w{3})',
    'j' => '(\d{1,2})',
    'l' => '(\w{6,9})',
    'F' => '(\w{3,9})',
    'm' => '(\d{2})',
    'M' => '(\w{3})',
    'n' => '(\d{1,2})',
    'o' => '(\d{4})',
    'Y' => '(\d{4})',
    'y' => '(\d{2})',
    'a' => '([ap]m)',
    'A' => '([AP]M)',
    'g' => '(\d{1,2})',
    'G' => '(\d{1,2})',
    'h' => '(\d{2})',
    'H' => '(\d{2})',
    'i' => '(\d{2})',
    's' => '(\d{2})',
  );

  $format_matches = array();

  // Get an array of characters that match a character the date() function can
  // understand.
  if (!preg_match_all('/(\w)/', $format, $format_matches)) {
    return FALSE;
  }

  // Build a regular expression based upon the format.
  $format = str_replace(array_keys($characters), array_values($characters), preg_quote($format));

  // Get an array of values that match the regular expression we built.
  if (!preg_match('/' . $format . '/', $time, $time_matches)) {
    return FALSE;
  }

  $i = 0;

  // We use $format_matches as a parallel array of $time_matches to get the
  // actual value of each part of the $time string.
  foreach ($format_matches[0] as $character) {
    ++$i;

    switch ($character) {
      case 'd':
      case 'j':
        $day = (int) $time_matches[$i];
        break;
      case 'm':
      case 'n':
        $month = (int) $time_matches[$i];
        break;
      case 'F':
      case 'M':
        $month_numbers = array(
          'Jan' => 1,
          'Feb' => 2,
          'Mar' => 3,
          'Apr' => 4,
          'May' => 5,
          'Jun' => 6,
          'Jul' => 7,
          'Aug' => 8,
          'Sep' => 9,
          'Oct' => 10,
          'Nov' => 11,
          'Dec' => 12,
        );

        $month = $month_numbers[substr($time_matches[$i], 0, 3)];
        break;
      case 'o':
      case 'Y':
      case 'y':
        $year = (int) $time_matches[$i];
        break;
      case 'a':
      case 'A':
        $meridiem = strtolower($time_matches[$i]);
        break;
      case 'g':
      case 'G':
      case 'h':
      case 'H':
        $hour = (int) $time_matches[$i];
        break;
      case 'i':
        $minute = (int) $time_matches[$i];
        break;
      case 's':
        $second = (int) $time_matches[$i];
        break;
    }
  }

  // These are required to create a Unix Timestamp with mktime().
  if (!isset($month) || !isset($day) || !isset($year)) {
    return FALSE;
  }

  // A time cannot be in 24 hour format with an AM or PM as well.
  if (isset($meridiem) && isset($hour) && $hour > 12) {
    return FALSE;
  }

  // If it's PM and not 12:##PM, add 12 hours for 24 hour format.
  if (isset($meridiem) && $meridiem == 'pm' && $hour < 12) {
    $hour += 12;
  }

  // If it's 12:##am, it's 00:## in 24 hour format.
  if (isset($meridiem) && $meridiem == 'am' && $hour == 12) {
    $hour = 0;
  }

  // If mktime() failed, at this point.. this function will fail.
  if (FALSE === $time = mktime(isset($hour) ? $hour : 0, isset($minute) ? $minute : 0, isset($second) ? $second : 0, $month, $day, $year)) {
    return FALSE;
  }

  return $time;
}