code: private function parseDateValueFormat($value, $format)
      {
          if (is_array($value)) {
              return false;
          }
          if (strncmp($format, 'php:', 4) === 0) {
              $format = substr($format, 4);
          } else {
              if (extension_loaded('intl')) {
                  return $this->parseDateValueIntl($value, $format);
              }
  
              // fallback to PHP if intl is not installed
              $format = FormatConverter::convertDateIcuToPhp($format, 'date');
          }
  
          return $this->parseDateValuePHP($value, $format);
      }
docs: Parses date string into UNIX timestamp.
  
  @param string $value string representing date
  @param string $format expected date format
  @return int|false a UNIX timestamp or `false` on failure.