bun.sh/docs/guides/runtime/timezone.md

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Set a time zone in Bun

Bun supports programmatically setting a default time zone for the lifetime of the bun process. To do set, set the value of the TZ environment variable to a valid timezone identifier.

{% callout %} When running a file with bun, the timezone defaults to your system's configured local time zone.

When running tests with bun test, the timezone is set to UTC to make tests more deterministic. {% /callout %}

process.env.TZ = "America/New_York";

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Alternatively, this can be set from the command line when running a Bun command.

$ TZ=America/New_York bun run dev

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Once TZ is set, any Date instances will have that time zone. By default all dates use your system's configured time zone.

new Date().getHours(); // => 18

process.env.TZ = "America/New_York";

new Date().getHours(); // => 21