Its not just your code that needs timezone awareness, users are all shockingly bad at understanding timezones. Daylight savings in particular needs to die.
The hardest I have noticed is to make them understand that it is not only the time, but also the date that change.
So when discussing whole timestamp or time - everybody is somewhat aware there is going to be timezone involved.
Good luck however, if you need to ask the question: "Is the instruction date (yyyy-mm-dd) in the Counterparty (Sidney), Party (London) or System timezone (Brussels) ?"
I think a more considered way to say that is it's a UX challange.
You have to think about what the user want's to enter and what they want to see. It's especially hard if they are doing something like booking a flight, or car hire in another country, how do you know if they are thinking about local time or destination time?
