Errors creating repeating event

Drupal 5.7, 3/12 version of date module, problem occurs both with PHP 5.2.0 on my Windows system and 5.2.1 on my Linux webhost. Enabled modules include Date API, Date Timezone, Date Popup, Date Repeat Date, CCK (no types other than Date enabled). I added a Datestamp field named Schedule to book pages with Repeating enabled, and No time zone conversion. Site timezone is US/Eastern.

When setting a biweekly repeat on a book node, after submitting the node I see these errors (once for each repeated instance):

# warning: __clone method called on non-object in /var/www/html/hrnemc/includes/common.inc on line 1385.
# warning: date_modify() expects parameter 1 to be DateTime, null given in /var/www/html/hrnemc/sites/all/modules/date/date/date_repeat.inc on line 117.
# warning: date_format() expects parameter 1 to be DateTime, null given in /var/www/html/hrnemc/sites/all/modules/date/date/date_repeat.inc on line 119.
# warning: date_format() expects parameter 1 to be DateTime, null given in /var/www/html/hrnemc/sites/all/modules/date/date/date_repeat.inc on line 120.
# warning: date_offset_get() expects parameter 1 to be DateTime, null given in /var/www/html/hrnemc/sites/all/modules/date/date/date_repeat.inc on line 121.
# warning: date_offset_get() expects parameter 1 to be DateTime, null given in /var/www/html/hrnemc/sites/all/modules/date/date/date_repeat.inc on line 122.

Viewing the node afterwards gives this error (it does not display the start date, but does display the repeat):

warning: date_format() expects parameter 1 to be DateTime, null given in /var/www/html/hrnemc/sites/all/modules/date/date_repeat/date_repeat_calc.inc on line 317.

Poking around the code, timezone_open() always seems to return an empty object, even if I hardcode 'UTC' as the parameter. I haven't been able to figure out why, though...