Why a Hardcoded List of Time Zone Offsets Eventually Starts Lying to You

"UTC+6" isn't always true

It's tempting to think of a time zone as a single fixed number — "Bangladesh is UTC+6, done." For zones without daylight saving time that's stable. But a large share of the world's population lives somewhere that shifts its clocks at least once a year, and the exact dates of that shift, or whether it happens at all, are decided by individual governments and can change with little warning.

Real changes, not hypothetical ones

Countries have added, removed, or shifted daylight saving rules many times over the decades — sometimes for energy policy reasons, sometimes reversed after a few years, sometimes changed for one region of a country but not another. A tool that hardcodes "this zone is UTC+X" as a permanent fact will be correct today and silently wrong the next time a government changes the rule, with no obvious warning to the person relying on it.

Where the correct data actually lives

The IANA Time Zone Database (often called "tzdata") is the standard reference almost every operating system and browser relies on for this. It's actively maintained specifically because these rules keep changing, and updates ship through regular OS and browser updates — meaning your device's own time zone data is more current than almost any hand-maintained table a web tool could embed.

Using the platform instead of a copy of it

Modern browsers expose this same, always-updated database directly to JavaScript through the Intl API — there's no need to maintain a separate copy that can drift out of sync with reality.

Try it yourself

Our World Clock / Time Zone Converter reads time zone rules directly from your browser's built-in database rather than a hardcoded table, so conversions stay correct even after a country changes its daylight saving rules — no update to this tool required.