Time zones are one of those topics that look simple until you ship something and a user in another country sees the wrong time. Here are the traps I keep seeing, and how to reason about them in 2026.
UTC is not a time zone, and GMT is not UTC
UTC (Coordinated Universal Time) is a time standard, not a region. GMT is a time zone that happens to share the same offset as UTC most of the year. For storage and math, always think in UTC. Treat GMT as just another named zone.
Rule 1: store timestamps in UTC
Store every instant as UTC (or an epoch value). Convert to a local zone only at the edges, when you display to a user. If you store local times, you will eventually lose the offset and never recover the true instant.
