<p>Even though the JavaDoc does not contain a hint about it, Calendars are inherently unsafe for multihtreaded use. 
Sharing a single instance across thread boundaries without proper synchronization will result in erratic behavior of the
application. Under 1.4 problems seem to surface less often than under Java 5 where you will probably see
random ArrayIndexOutOfBoundsExceptions or IndexOutOfBoundsExceptions in sun.util.calendar.BaseCalendar.getCalendarDateFromFixedDate().</p>
<p>You may also experience serialization problems.</p>
<p>Using an instance field is recommended.</p>
<p>For more information on this see <a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6231579">Sun Bug #6231579</a>
and <a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6178997">Sun Bug #6178997</a>.</p>