If I understand you correctly, you are not able to fix this, because its a Posterous "bug" (or better said missing feature). I already mailed the Posterous support when they will support different time zones. They answered that they already working on it.
It's actually a clumsy interface in the API... You are correct all times on Posterous are in 1 single time zone, but that is not my issue here. It's the API that uses a difficult to handle time format