To make a page see a fixed JavaScript date, register a Date override with page.evaluateOnNewDocument() before navigating. If you need timers and browser scheduling to use a synthetic clock, use Chrome DevTools Protocol (CDP) virtual time instead. If you need local-time formatting in a particular zone, set that separately with page.emulateTimezone().
Choose the right kind of date override
These techniques affect different things. A page-level patch controls JavaScript Date calls in the page context. CDP virtual time supplies a synthetic clock for browser frames and scheduler behavior. Timezone emulation changes how local time is interpreted; it does not choose a new instant.
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| Need | Use | What it changes |
|---|---|---|
| Make startup JavaScript read a specific date | page.evaluateOnNewDocument() with a Date patch |
No-argument new Date() and Date.now() in the patched page context |
| Run timer and scheduler work against synthetic time | CDP Emulation.setVirtualTimePolicy |
The browser’s virtual-time source for frames; behavior depends on the selected policy and budget |
| Test local date or time rendering in a zone | page.emulateTimezone() |
Timezone behavior, not the underlying instant |
Puppeteer documents page.evaluate() as evaluating a function in the page’s context. That is useful for inspecting or changing an already-loaded page, but it can be too late for startup code that has already read or cached date values. The pre-script hook is page.evaluateOnNewDocument().
Fix JavaScript Date before the site runs
Register the override before page.goto(). This example fixes no-argument date construction and Date.now(), while preserving explicit arguments such as new Date('2025-01-01').
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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 timestamp is calculated in Node.js and passed to the page function through Puppeteer’s argument mechanism. The hook runs before the page’s scripts and also applies to child frames when they are attached or navigated, as described in the Puppeteer API reference.
This is a page-JavaScript patch, not a guarantee that every worker, isolated realm, or browser-native API sees a frozen wall clock. Check those contexts separately if they are part of the test. It also does not make timers advance against a synthetic scheduler clock.
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Use CDP virtual time for timers and scheduler behavior
For a synthetic browser clock rather than just patched JavaScript date reads, create a CDP session and send Emulation.setVirtualTimePolicy. Puppeteer’s Page API lists createCDPSession(). The protocol describes virtual time as replacing real time with a synthetic time source for all frames.
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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 protocol parameter initialVirtualTime is in epoch seconds. JavaScript’s Date.parse() returns milliseconds, hence the division by 1000. The budget is virtual milliseconds; when it expires, virtual time pauses and the protocol emits virtualTimeBudgetExpired. The available policies include advance, pause, and pauseIfNetworkFetchesPending.
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Placement and policy matter. Some tests need the policy set before navigation; others need a bounded advance after the page loads. Verify the sequence with the exact Chromium build and policy your test requires. The Chrome DevTools Protocol Emulation documentation marks this command experimental, so do not assume identical behavior across browser versions. Close or detach the CDP session when the test is finished, following the lifecycle supported by your Puppeteer version.
Set the timezone separately
When assertions depend on local clock methods or date formatting, set a timezone independently:
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await page.emulateTimezone('America/Los_Angeles');Puppeteer’s emulateTimezone() API changes the page timezone; passing null disables the emulation. Apply timezone and clock settings before navigation if startup code depends on them. A fixed instant can render as different local calendar dates in different zones, especially near midnight; changing the zone does not change that instant.
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- Overriding after navigation:
page.evaluate()may run after the site has read or cached date functions. Register the patch withevaluateOnNewDocument()beforegoto(). - Using timezone emulation as a clock:
emulateTimezone()changes timezone behavior, not the instant. Combine it with a date patch or virtual time if both are needed. - Expecting a Date patch to advance timers: The patch controls page JavaScript date reads; it does not set CDP’s scheduler policy. Use virtual time when timer progression is part of the test, and verify the Chromium behavior.
- Missing code in a frame: Use the pre-navigation hook for child frames, or virtual time for the browser-wide frame clock as supported by the protocol. Check any worker or separate realm explicitly.
- Using the wrong virtual-time units:
initialVirtualTimeis epoch seconds, not JavaScript milliseconds; convert aDate.parse()result by dividing by 1000. The budget is virtual milliseconds. - Assuming CDP compatibility: The virtual-time command is experimental. Check it against the Chrome or Chromium version actually used in CI and adjust the policy or setup sequence if needed.
Version and reliability considerations
Puppeteer documentation versions surfaced for these APIs include 25.10.0 through 25.12.0; the CDP page cited here is the current “tot” protocol reference, and labels virtual-time policy experimental. These pages do not establish that a given command behaves identically in every Puppeteer/Chromium pairing. Check the API reference and protocol supported by the versions installed in your project, then assert both the page’s observed date and the timer behavior your test depends on.
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Or skip the browser setup
If your goal is to capture a page rather than test its date logic, ScreenshotNeo is a website screenshot API and MCP server. It does not replace Puppeteer date mocking, but a single request can capture a URL without setting up a browser locally.
See the ScreenshotNeo API documentation for request options. Example cURL call:
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Does overriding Date change the server’s date?
No. These techniques affect the browser-side test environment; they do not change the clock used by the website’s server.
Can I combine a fixed instant with a timezone?
Yes. Set the instant with a page patch or CDP virtual time, and set local-time behavior independently with timezone emulation.
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