How to set Chrome's time zone in Selenium tests

For Selenium tests, your browser starts in your local timezone, or whatever your system's environment specifies.
This is usually good enough. To test any timezone-dependent behavior in Chrome, you can change the time zone using the Chrome DevTools Protocol Show archive.org snapshot .

Example

Chrome accepts any IANA time zone name, like "Europe/Berlin" or "Asia/Tokyo".
Here is the relevant command for Capybara:

Copypage.driver.browser.execute_cdp('Emulation.setTimezoneOverride', timezoneId: 'Asia/Tokyo')

Important: This change is permanent throughout the browser session. You need to remove the time zone override at the end of each test explicitly, or it affects other tests.
The CDP specifies that resetting works by setting an empty time zone name Show archive.org snapshot .

Capybara helper

I suggest you wrap this into a helper method for your Capybara tests, like so:

Copymodule TimeZoneHelpers

  def set_browser_time_zone(iana_zone_name)
    # Expects an IANA time zone name, like "Europe/Berlin" or "Asia/Tokyo".
    page.driver.browser.execute_cdp('Emulation.setTimezoneOverride', timezoneId: iana_zone_name)
  end
  
end

RSpec.configure do |config|
  config.include(TimeZoneHelpers, type: :feature)

  config.after(type: :feature, js: true) do
    set_browser_time_zone('')
  end
end

Your tests can then use set_browser_time_zone and any changes will be reverted after each test.