warning: timezone_open() [function.timezone-open]: Unknown or bad timezone (UTC)

Recently upgraded to PHP5.3 and started seeing errors:

• warning: timezone_open() [function.timezone-open]: Unknown or bad timezone (UTC) in /home/truhome/public_html/new/sites/all/modules/calendar/calendar.module on line 614.
• warning: date_timezone_set() expects parameter 2 to be DateTimeZone, boolean given in /home/truhome/public_html/new/sites/all/modules/calendar/calendar.module on line 614.
• warning: timezone_open() [function.timezone-open]: Unknown or bad timezone (UTC) in /home/truhome/public_html/new/sites/all/modules/calendar/calendar.module on line 614.
• warning: date_timezone_set() expects parameter 2 to be DateTimeZone, boolean given in /home/truhome/public_html/new/sites/all/modules/calendar/calendar.module on line 614.

No idea why this is happening. Browsing to line 615 of calendar.module we see this:

  // Translate the view min and max dates to UTC values
  // so we can compare UTC dates to the view range.
  $min_utc = drupal_clone($view->date_info->min_date);
  date_timezone_set($min_utc, timezone_open('UTC'));
  $max_utc = drupal_clone($view->date_info->max_date);
  date_timezone_set($max_utc, timezone_open('UTC'));
...

Fair enough, looks like UTC is hard-coded. Changing the 'UTC' string to a different timezone (say, "America/Vancouver") makes all those errors go away.

I suspect PHP is at fault. Ideas?