To make a page see a fixed JavaScript date, install a Date override with page.evaluateOnNewDocument() before navigating. If your test also needs timers and browser scheduling to use synthetic time, use Chrome DevTools Protocol (CDP) virtual time instead. Set the timezone separately when local-time methods or formatting matter: timezone emulation changes how an instant is interpreted, not the instant itself.
Choose the right kind of date override
| Need | Use | What it controls |
|---|---|---|
Make page code that reads JavaScript Date see a fixed instant |
page.evaluateOnNewDocument() with a Date patch |
The page JavaScript environment where the override is installed. It does not establish that every browser subsystem, worker, or native API uses a frozen wall clock. |
| Run timers and scheduler work against synthetic time | CDP Emulation.setVirtualTimePolicy |
A synthetic time source for browser frames. The protocol currently marks this command experimental, so check it against the Chromium version used in your test. |
| Test local clock methods or date formatting in a particular region | page.emulateTimezone() |
Timezone interpretation and rendering; it does not set the clock’s instant. |
Puppeteer describes page.evaluate() as evaluating a function in the existing page context and returning its result. That is too late if startup scripts have already read or cached date values. The pre-navigation hook is the appropriate page-level option when the site must encounter the override before its scripts execute. See the Puppeteer Page.evaluate() API and Page.evaluateOnNewDocument() API.
Fix JavaScript Date before the page loads
Register the override before goto(). The sample fixes no-argument new Date() and Date.now(), while preserving the original constructor’s behavior for calls that pass arguments.
const fixedTime = new Date('2030-01-02T03:04:05.000Z').valueOf();
await page.evaluateOnNewDocument((timestamp) => {
const OriginalDate = Date;
class FixedDate extends OriginalDate {
constructor(...args) {
if (args.length === 0) {
super(timestamp);
} else {
super(...args);
}
}
static now() {
return timestamp;
}
}
Object.setPrototypeOf(FixedDate, OriginalDate);
globalThis.Date = FixedDate;
}, fixedTime);
await page.goto('https://example.test');The argument passed to evaluateOnNewDocument() supplies the timestamp to the page-side function. The hook runs before page scripts and also applies when child frames are attached or navigated, according to Puppeteer’s API documentation. The patch is still a JavaScript-level technique: check separately if your test relies on workers, isolated realms, or code that cached Date in another environment.
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Use CDP virtual time for timers and scheduler behavior
When a test needs browser scheduling and timers to progress against a synthetic clock, create a CDP session and set virtual time. The protocol says this command turns on virtual time for all frames, replacing real time with a synthetic source. Its initialVirtualTime parameter is expressed in seconds since the epoch; JavaScript’s Date.parse() returns milliseconds, so divide by 1,000.
const client = await page.createCDPSession();
await client.send('Emulation.setVirtualTimePolicy', {
policy: 'pauseIfNetworkFetchesPending',
initialVirtualTime: Date.parse('2030-01-02T03:04:05.000Z') / 1000,
budget: 5000,
});
await page.goto('https://example.test');The policy may be advance, pause, or pauseIfNetworkFetchesPending. With a budget, virtual time pauses after the specified virtual milliseconds and emits virtualTimeBudgetExpired. This code is a starting pattern, not a browser-independent guarantee: confirm policy timing and behavior for the Puppeteer and Chromium versions in your CI. Some tests need the policy before navigation; others need a bounded period of virtual advancement after the page loads. The CDP reference labels Emulation.setVirtualTimePolicy experimental. See the Chrome DevTools Protocol Emulation domain and Puppeteer’s Page.createCDPSession() API. Close or detach the session when the test is finished, following the lifecycle supported by your installed Puppeteer version.
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Set timezone independently
If assertions use local-time methods or formatted output, set the timezone as well as the clock behavior:
await page.emulateTimezone('America/Los_Angeles');Puppeteer documents emulateTimezone(timezoneId) for changing the page timezone; passing null disables emulation. Apply it before navigation if startup code depends on the zone. A fixed instant can render as different local calendar dates near midnight in different timezones; that reflects timezone interpretation, not a changed instant. See the Puppeteer Page.emulateTimezone() API.
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Common failures and how to fix them
- The page still shows the real date. If the override ran through
page.evaluate()after navigation, startup code may already have read or cached the value. Register the patch withevaluateOnNewDocument()beforegoto(). - The date is fixed, but timers do not behave as expected. Patching JavaScript
Datedoes not configure the browser scheduler. Use CDP virtual time and verify its policy and timing against your Chromium build. - The displayed calendar day is unexpected. Check the selected timezone and whether the asserted value is UTC or local time. Timezone emulation and the chosen instant are separate settings.
- A frame sees a different value. Puppeteer’s pre-script hook is documented to run for child frames when attached or navigated, while CDP virtual time applies to all frames. Check whether the code instead runs in a worker, isolated realm, or context that cached the original constructor.
- CDP rejects the command or behavior differs in CI. The virtual-time command is experimental. Check the protocol supported by the exact Chrome/Chromium build and the Puppeteer version installed by the project.
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