Its not just your code that needs timezone awareness, users are all shockingly bad at understanding timezones. Daylight savings in particular needs to die.
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) ?"
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?