Playwright timezone does not match the proxy IP

You set timezone_id. You are still flagged for a timezone mismatch. This page is why that happens, and it is usually not the setting being ignored.

The short version: the timezone is not one value, and neither is the locale. They are half a dozen surfaces, fed by different mechanisms inside the browser, and a detector compares them with each other and with your exit IP. Setting one of them correctly leaves the others answering the old question.

Everything that has to agree

Run this on any page and read all of it, not the first line:

Intl.DateTimeFormat().resolvedOptions().timeZone   // "America/New_York"
new Date().getTimezoneOffset()                      // 300  (minutes, inverted)
new Date().toString()                               // "... GMT-0500 (Eastern Standard Time)"
navigator.language                                  // "en-US"
navigator.languages                                 // ["en-US", "en"]
new Intl.NumberFormat().format(1234.5)              // "1,234.5"  or  "1.234,5"
new Date().toLocaleString()                         // month names, order, separators

Plus two things that are not in JavaScript at all:

the Accept-Language header your requests actually carry
the country of the IP the request left from

Nine values. A mismatch check does not need any of them to be strange. It needs two of them to disagree.

The combinations that get caught most often:

Proxy in one country, timezone from your own machine. The classic. Your host is in Europe, your exit is in the United States, and you never set anything.
Timezone set, locale not. A US exit, America/New_York, and de-DE in navigator.languages because that is what the machine had.
Intl output disagreeing with navigator.language. These are genuinely separate mechanisms in the browser, and one can be set while the other is not. Read both.
Accept-Language disagreeing with navigator.languages. Same list, two places, and the header is often set by a different layer than the browser property.
The offset not matching the zone on that date. America/New_York is -300 in January and -240 in July. A hardcoded offset is right for half the year.

Why the environment variable does not work

A common suggestion is to set TZ before launching the browser. It is worth knowing precisely when that fails.

On Windows, Firefox ignores TZ unless it is in POSIX form, like EST5EDT. The IANA identifier you actually want, America/New_York, is not recognised there. So the variable appears to be set, the browser reports something else, and the usual conclusion is that the setting was ignored.

The mechanism that does work in current Firefox is a per-realm override applied by the engine, which is what Playwright’s timezone_id reaches. That is also why the value has to be re-applied to realms created later: a new page, an iframe on another site, a worker. Under site isolation each new site can be a new process, and anything not re-applied there falls back to the build’s default.

If you are debugging this, check the value inside an iframe from a different origin, not just on the top page. That is where a partially applied override shows up, and it is the check most people never run.

Derive it from the exit, do not configure it

The structural fix is to stop treating the timezone as configuration and start treating it as a consequence of the proxy.

If you hardcode America/New_York and your proxy pool rotates through three countries, you are wrong two thirds of the time, and you are wrong in the specific way that gets looked for. The zone has to be resolved from the address the session actually leaves from, at launch, every time.

How this project does it, because the details are the part that bites:

One lookup discovers the egress IP through the proxy, not from the host. A request that does not go through the proxy tells you about your own connection.
That IP maps to a zone with an offline database, so there is no third-party request per launch announcing that a new session started.
The same lookup feeds the locale and the WebRTC public address, so those three cannot drift apart. They come from one answer, not three.
On failure with a proxy configured, it fails the launch instead of falling back to the host timezone. That fallback is the exact mismatch you set out to avoid, and a silent fallback to it is worse than an error.

That last one is a small decision that took a while to get right. A fallback that “keeps things working” is the wrong default when the thing it falls back to is the bug.

Country-level is as good as it gets, and that is usually fine

IP-to-location databases are reliable at country level and much less so below it. One country gets one zone, which means a US exit becomes a single American zone rather than the correct one of six, and Belgium gets one of its two languages.

This is fine and worth saying out loud, because it sets a limit on how much effort is worth spending. Nobody is checking whether your zone is the right one within the country. They are checking whether it is in the right country at all. Pick a plausible zone for the country and stop.

Checking it

Through the proxy you actually deploy with, on a real page:

Read all nine values above in one go.
Check the country of your exit IP separately.
Confirm the offset is correct for today’s date, not in general.
Repeat inside a cross-origin iframe.
Run the same page on a stock browser on a machine in that country, and diff.

Step five is the one that catches what you did not think to check.

Short answers to the questions that lead here

Does timezone_id in Playwright actually work? Yes, for the timezone. It does not touch the locale, the Accept-Language header, or your exit IP.

Do I need to set locale too? Yes, and match it to the same country. A US exit with a German locale is one of the most commonly detected combinations there is.

Can I just use UTC? You can, and almost nobody is genuinely in UTC. It is a small signal on its own and an obvious one in combination with a residential IP.

Why does it work headed and fail headless? Usually not headless. Usually the two runs left from different addresses.

Why does the offset look inverted? getTimezoneOffset() returns minutes to add to local time to get UTC, so zones west of Greenwich are positive. It is not a bug and it is a common source of an hour’s confusion.

Is a VPN better than a proxy here? For this specific problem they are equivalent, as long as you resolve the zone from the address you actually exit through.

See also: Firefox preferences that silently do nothing, if you are here because a setting looks ignored, WebRTC leak with a proxy, which is the other half of “the browser and the network disagree”, and the checklist for being detected on one site.

From the notes of invisible_playwright. The Windows TZ behaviour above is something we found the slow way, after concluding several times that the setting was being ignored.