Writing Date and Time Code With LLMs in November 2026: Store UTC, Name the Time Zone, and Test the Daylight Saving Edges
Date and time code looks simple until it breaks. A report counts one day twice, a reminder fires an hour late, or a nightly job runs zero times on the night the clocks change. LLMs are good at writing date and time code quickly, but they tend to write the happy path: one time zone, no daylight saving change, and a server clock that matches the user's. This guide shows how to get date and time code from a model that holds up in production. The short version: store instants in UTC, keep the user's named time zone next to anything that is really a local time, and test the days when the clock jumps. The rest of this post explains each step with examples you can copy. One instant, several wall-clock readings. Your code needs to know which one it is holding. Image: Clocks London, Berlin, Jersey. Jersey War Tunnels, 2009 by fw190a8, CC BY-SA 2.0 , via Wikimedia Commons. Why models get time wrong Most date bugs come from mixing up two different things: An instant : a single...