Birthdays and Timezone

[gbac]

I shifted timezone (TZ2). everything's cool. when I shifted back to
original timezone (TZ1), all birthdays have shifted to TZ2.

1. Why does this happen?
(If this is a normal outlook behaviour, it would be hell for people
like me who travel a lot.)

2. How do I prevent this from happening?

3. How do I know shift all birtydays back to original timezone?

Thanks