Timezone map in date_php4_tz_map.inc probably wrong

updating my site to current date api 2,
i get these errors when i display a calendar block showing event.module events.

# warning: date_timezone_set() expects parameter 2 to be DateTimeZone, null given in /www/htdocs/konzep/z/sites/all/modules/calendar/calendar.inc on line 628.
# warning: date_timezone_set() expects parameter 2 to be DateTimeZone, null given in /www/htdocs/konzep/z/sites/all/modules/calendar/calendar.inc on line 629.

the offending codeblock is:

<?php
                  $timezone = calendar_event_timezone($node->event_timezone);
                  date_timezone_set($node->calendar_start_date, $timezone);
                  date_timezone_set($node->calendar_end_date, $timezone);
?>

with

<?php
/**
 * Get the timezone name from an Event module timezone zid.
 */
function calendar_event_timezone($zid) {
  static $timezones;
  if (empty($timezones)) {
    // The array that maps event timezone zids to timezone names is in
    // date_php4_tz_map.inc, need to reverse it so the zid is the key.
    include_once(drupal_get_path('module', 'date_php4') .'/date_php4_tz_map.inc');
    $timezones = array('' => '');
    $map = $timezone_map;
    foreach ($map as $zone => $values) {
      if (!empty($values['zid'])) {
        $timezones[$values['zid']] = $zone;
      }
    }
  }
  return $timezones[$zid];
}
?>

some debugging tells me that $zid = 314
and $timezones:

(
[] => {empty string}
[303] => US/Hawaii
[313] => US/Alaska
[308] => Etc/GMT-4
[306] => Etc/GMT-2
[307] => Etc/GMT-3
[310] => US/Central
[309] => US/Michigan
[311] => US/Mountain
[312] => US/Pacific
[302] => Etc/GMT-1
[143] => Canada/Newfoundland
[304] => US/Samoa
[305] => Etc/GMT-12
[486] => Pacific/Marquesas
)

this is not enough.
HOOPS, in date_php4_tz_map.inc many entries for zid are zero. this should not be.

changing "my" timezone to 314 solves the issue for me,
after apoplying #235434: warning: date_format() expects parameter 1 to be DateTime, null given also