Why do we still trust LLMs to output raw JSON or dates?

I had my agent hallucinate the wrong day of the week three times in a row simply because I let it calculate timestamps on its own instead of using a basic system script. Moving all structured data generation like JSON fields file paths and dates away from the model and into deterministic Python scripts completely fixed our pipeline stability.

LLMs are great for writing plain text and copy but terrible at programmatic math. Do you still let your agents handle raw config values or do you lock those down with scripts?