# no-timezone-without-explicit-timezone

> Without the Intl.DateTimeFormat timeZone option, dates format in the server's zone and then in the browser's, so hydration fails. This rule reports it.

Source: https://hydration.jscrate.dev/docs/rules/no-timezone-without-explicit-timezone
Last updated: 2026-09-18

Without the Intl.DateTimeFormat timeZone option, a date is formatted in the
time zone of whichever runtime renders it: UTC on most servers, the visitor's
zone in the browser. `no-timezone-without-explicit-timezone` reports date
formatting without an explicit `timeZone`, and local-time getters such as
`getHours()`, in render code.

| | |
| --- | --- |
| Rule | `hydration-proof/no-timezone-without-explicit-timezone` |
| What it reports | Require an explicit timeZone when dates are formatted or split into parts during render |
| recommended / next | Warning |
| strict | Error |
| Server Components | Skipped with the next preset |
| Suggestions | Yes |
| Options | defaultTimeZone |

## What it reports

In [render code](https://hydration.jscrate.dev/docs/eslint#what-counts-as-render):

- `toLocaleDateString()` and `toLocaleTimeString()` without a `timeZone`
  option.
- `new Intl.DateTimeFormat()` / `Intl.DateTimeFormat()` without a `timeZone`
  option.
- `toLocaleString()` without a `timeZone` option when the value is clearly a
  date: the receiver is `new Date(...)` (or a variable initialized with it in
  the same function), or the options contain date or time fields
  (`dateStyle`, `timeStyle`, `year`, `month`, `day`, `hour`, `minute`, ...).
- `getHours()`, `getMinutes()`, `getDate()`, `getDay()`, `getMonth()`,
  `getFullYear()`, `getTimezoneOffset()`, `toDateString()`, `toTimeString()`
  and `toString()` on `new Date(...)` or a variable initialized with it in the
  same function.
- `Intl.DateTimeFormat().resolvedOptions().timeZone`, which reads the
  runtime's time zone.

When the options are not an object literal
(`toLocaleDateString('en-US', options)`), the rule cannot see them and does
not report.

Suggestions (never automatic fixes):

- add `timeZone: 'UTC'` (or the `defaultTimeZone` option) to the options
  object, or add an options object when there is none;
- replace `getHours()` and the other local getters with their UTC versions
  (`getUTCHours()`), when `defaultTimeZone` is `'UTC'`.

## Why the Intl.DateTimeFormat timeZone option matters

Dates are formatted in the time zone of the runtime. Servers usually run in
UTC, browsers in the visitor's zone:

```text
server HTML:   <p>Signed in at 5:00 AM</p>    (UTC)
client render: <p>Signed in at 10:00 AM</p>   (Asia/Karachi)
```

Hydration fails whenever the two zones give a different result, which for
times is almost always, and for dates near midnight.
[Timezone hydration mismatches](https://hydration.jscrate.dev/docs/causes/timezone) explains how to pick
the zone both sides use.

## Incorrect

```jsx
function LastLogin({ at }) {
  return <p>Signed in at {new Date(at).toLocaleTimeString("en-US")}</p>;
}

function Posted({ at }) {
  const date = new Date(at);
  return (
    <p>
      {date.getDate()}/{date.getMonth() + 1}
    </p>
  );
}

function Schedule({ at }) {
  const format = new Intl.DateTimeFormat("en-US", {
    dateStyle: "medium",
    timeStyle: "short",
  });
  return <p>{format.format(at)}</p>;
}
```

## Correct

```jsx
function LastLogin({ at, timeZone }) {
  // The user's zone, stored in their profile or a cookie and passed from the server.
  return (
    <p>Signed in at {new Date(at).toLocaleTimeString("en-US", { timeZone })}</p>
  );
}

function Posted({ at }) {
  const date = new Date(at);
  return (
    <p>
      {date.getUTCDate()}/{date.getUTCMonth() + 1}
    </p>
  );
}

function Schedule({ at }) {
  const format = new Intl.DateTimeFormat("en-US", {
    timeZone: "UTC",
    dateStyle: "medium",
    timeStyle: "short",
  });
  return <p>{format.format(at)}</p>;
}
```

If the visitor's own zone is required and the server cannot know it, render
the date after hydration (in `useEffect`) or put `suppressHydrationWarning` on
the element that shows it.

## Options

| Option | Type | Default | Description |
| --- | --- | --- | --- |
| `defaultTimeZone` | `string` | `'UTC'` | Time zone inserted by the suggestion. |

```js title="eslint.config.mjs"
{
  rules: {
    'hydration-proof/no-timezone-without-explicit-timezone': ['warn', { defaultTimeZone: 'Europe/Berlin' }],
  },
}
```

- `defaultTimeZone` (string, default `'UTC'`): the zone the suggestion
  inserts. The UTC getter suggestion is only offered when this is `'UTC'`.

## Messages

What ESLint prints for this rule, word for word:

- `<call>` formats the date in the time zone of whichever runtime renders it. Servers usually run in UTC and browsers in the visitor's zone, so the text does not match during hydration. Pass an explicit `timeZone` option.
- `<call>` reads the date in the runtime's local time zone, which differs between the server and the browser. Use the UTC methods, or format with an explicit `timeZone`.
- `<call>` reads the runtime's time zone, which differs between the server and the browser. Pass the time zone from the server, or read it after hydration.
- Add timeZone: '<timeZone>'.
- Use <method>() (UTC).

## When not to use it

When the servers and all visitors share one time zone, or the runtime's
default zone is set explicitly on both sides.

## Related

- [`no-locale-without-explicit-locale`](https://hydration.jscrate.dev/docs/rules/no-locale-without-explicit-locale):
  `date.toLocaleDateString()` without arguments is missing a locale and a time
  zone. Both rules report it: this one on the method name, that one on the
  whole call. They are two separate fixes.
- [`no-date-in-render`](https://hydration.jscrate.dev/docs/rules/no-date-in-render): `new Date().getHours()`
  reads the current time (that rule) and splits it in the local zone (this
  rule).
- [HP1001: text differs between server and client](https://hydration.jscrate.dev/docs/issues/hp1001), how
  `hydration-proof test` reports a timezone difference
- [Timezone hydration mismatches](https://hydration.jscrate.dev/docs/causes/timezone): the cause and its
  fixes