Add guides for test runner (#4308)

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diff --git a/docs/guides/runtime/timezone.md b/docs/guides/runtime/timezone.md
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+---
+name: 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](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones).
+
+{% 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 %}
+
+```ts
+process.env.TZ = "America/New_York";
+```
+
+---
+
+Alternatively, this can be set from the command line when running a Bun command.
+
+```sh
+$ TZ=America/New_York bun run dev
+```
+
+---
+
+Once `TZ` is set, any `Date` instances will have that time zone. By default all dates use your system's configured time zone.
+
+```ts
+new Date().getHours(); // => 18
+
+process.env.TZ = "America/New_York";
+
+new Date().getHours(); // => 21
+```