If you use the date/time picker in issue navigator and use the "before" field, they do not work properly. Here is an example:
Tickert create on October 26. 2010
Try query: "Before October 27 2010" shows the ticket
Try query: "Before October 26 2010" does not show the ticket, alhtough the mouseover tells, "shows tickets before or on this date"

The two screenshots show the sample made on the Jira system on jira.atlassian.com
The same happens with out locale installation