This is where you would go wrong here we dont have a time offsets for guests. it will just use the time of the server.
i think your over analyzing your time thing personally if I understand your time calculations ABOVE.
Your
vb time and Photopost time should all be set to use 12 for aukland as the base timezone. Then members set their own timezone different from aukland and things should all be fine.
I think your issue is coming from the fact your trying to do 2 different time calculations. One with +1 from GMT and one from +12
You can only set Photopost to whatever time you wish and then users offsets get applied from there.
Therefore if your users are setting their time offsets from +1 GMT instead of Aukland +12 I can see where you would have an issue.