Hi (again),
  I forgot to mention in my previous posting, that there could also be a 
boolean dst attribute, for Day Light Savings.
eg.
<datetime calendar="gregorian" locale="au" tz="+10:00" 
dst="dst">...</datetime>
Note: the timezone is still specified as the normal +10:00, rather than 
+11:00 (for eastern Australia) because the dst attribute tells the user 
agent that this time has been written in day light savings, and can 
therefore subtract an hour from the given time, before further conversions.
CYA
...Lachy