Improve timestamp substraction to be DST-aware

Hi,

I would like to implement item from TODO list: "Improve TIMESTAMP WITH
TIME ZONE subtraction to be DST-aware".

To compute interval properly, we need time zone. Currently in timestamp
we don't have time zone information, so I would use actual session time
zone for computations. This can cause problems in the future, when TZ
info will be saved in DB (this is also on TODO list). In such case, user
may suspect, that stored TZ will be used, and probably behavior of
subtraction will change. Anyway, I think, that using session TZ now is
better than not using it at all.

Can I start with implementation?

Best regards
Piotr Marcinczyk