Bun Intl.DateTimeFormat timeZone support and bundled tzdata update policy

Short answer

Bun ships its own copy of the IANA Time‑Zone Database (tzdata) inside the runtime, independent of the host operating system. The bundled version is locked to the specific Bun release you run; updating the Bun binary is the only way to get a newer tzdata set. When parsing an ISO‑8601 string that lacks an explicit offset, Bun treats it as a local‑time value in the configured time zone. During a DST overlap or gap, the underlying JavaScriptCore implementation resolves the ambiguous instant by selecting the earlier or later offset according to its own algorithm (typically the earlier offset, but this is not guaranteed and can differ between JSC versions).

What we know

• Bundled data: Bun bundles tzdata to guarantee consistent formatting across platforms. It does not fall back to the host OS’s time‑zone database.
• Version tie‑in: Each Bun release includes a specific tzdata snapshot. The snapshot is updated only when Bun itself is released; there is no separate auto‑update mechanism.
• Parsing without offset: ISO‑8601 strings without a trailing Z or explicit offset are parsed as local time in the current time zone. The resulting Date object will represent the instant that the host environment would interpret.
• Ambiguous times: For times that fall into a DST overlap or gap, JavaScriptCore applies a deterministic rule (usually choosing the earlier offset). The rule is defined by the ECMA‑402 spec through the underlying engine and is not exposed as a configurable option in Bun.

How to verify on your machine

1. Check the bundled tzdata version:

   node -p "process.versions.tzdata"

   or, if process.versions.tzdata is not available, run:

   node -p "require('fs').readFileSync(process.execPath + '/tzdata.json', 'utf8').slice(0, 30)"

   Both commands will print the tzdata snapshot identifier that ships with the binary.

2. Test a known DST transition date in two different Bun releases (e.g., 2023‑11‑05T01:30:00 in America/New_York) and compare the offsets. The difference in output indicates a tzdata update.

3. Parse a local ISO‑8601 string during a DST overlap:

   new Date('2023-11-05T01:30:00') // no offset
   console.log(new Date('2023-11-05T01:30:00').toString());

   Observe which offset the runtime selects. Re‑run the code after upgrading Bun to see if the rule changes.

When to consider an upgrade

If your application depends on the most recent time‑zone rules (e.g., handling new DST changes or country‑specific adjustments), you should keep Bun up‑to‑date. The release notes for each Bun version usually list the tzdata snapshot used, so you can plan upgrades when a new snapshot is published.

Missing diagnostic detail

To give a more precise recommendation for your environment, it would help to know which Bun version you are running (e.g., bun --version output). If you’re on an older release, the bundled tzdata may be several years out‑of‑date.