Date off by one day in dateblock

I think this may be the same problem that was reported here (and said to be fixed): https://drupal.org/node/1180080

I'm very new to Drupal, so if I'm missing something obvious, please let me know.

In the dateblock only (both the node and calendar are correct), events that start after 5 pm show up in the dateblock one day later than they should be. Please see my screenshot in which an event scheduled for September 9, 2013 is correct in the node, but incorrect in the dateblock.

Date settings are set with Monday as the first day of the week. Site's timezone is America/Los Angeles. The site is running Drupal Core 6.28.

Thank you in advance for any help you can offer.