Drupal 7: date format conversion

my_common.inc

<?php

/**
 * Date conversion class.
 */
class DateFormat {

  /**
   * Date format conversion.
   *
   * Convert a date from any string format (accordingly with PHP standard) or
   * from Drupal array date format to the specified format (default: Y-m-d)
   *
   * @param string|array $in_date
   *   Date to be formatted, can be different values:
   *   - a string in any PHP standard format
   *   - an array 'Drupal style': ['year' => 2015, 'month' => 11, 'day' => 25]
   *   - 'now' to get current date.
   * @param string $format
   *   Any PHP supported Date and Time Formats (ex: 'timestamp', 'today', etc.)
   *   (see http://php.net/manual/en/datetime.formats.php).
   * @param string $default
   *   Returning value if some errors occurs during conversion:
   *   - 'now': return the current date
   *   - (default is an empty value).
   *
   * @return string
   *   The formatted date; if there is an error during conversion,
   *   an empty value is returned
   */
  public static function convert($in_date, $format = 'Y-m-d', $default = '') {
    if (empty($in_date) || is_null($in_date)) {
      return '';
    }
    elseif (is_string($in_date)) {
      $date_time = new DateTime($in_date);
    }
    elseif (is_array($in_date)) {
      $date_time = new DateTime(implode('-', $in_date));
    }

    if ($date_time == FALSE) {
      // Conversion error.
      switch ($default) {
        case 'now':
          $date_time = new DateTime();
          break;

        default:
          return '';
      }
    }

    if (strtolower($format) == 'timestamp') {
      $out_date = $date_time->getTimestamp();
    }
    else {
      $out_date = $date_time->format($format);
    }

    return $out_date;
  }

}