That's interesting. I'd recommend just doing the heavy lifting on behalf of the agent so that by the time it reaches the agent there's only like 2 options (Maaaybe 3... But probably not) that it shoots back to the user based on their preferences.
Also don't have the context but if it's appointment booking for sales, you'd probably want to have them book as early as possible anyways.
We pass a "now" field via make.com either at the onset, as part of the transient assistant config or as a webhook response upon tool use. Actually has never posed issues in terms of booking in the wrong year. That I'd usually point out to be a not-great solution to let the agent to even have the ability to book for 2023, instead, if that makes sense.