How to use a custom time in browser to test for client vs server time difference

On popular demand (kidding!), the explanation.

Google Chrome reads the environment variables to get the timezone its running in. Apparently all instances of Chrome share the value (not tested). In order to force a timezone, we need to a) set the environment variable to the timezone we want, b) kill all existing instances of Chrome, c) Print the current timezone to confirm, and d) Start Chrome with the intended timezone.

The below shell script is tested with OS X but should work for others too.

export TZ='US/Pacific'

killall Chrome
date
open /Applications/Google\ Chrome.app

Create a new empty directory for a separate Chrome user profile. E.g. with

mkdir ~/chrome-profile

You specify the TZ environment variable. You can see the valid timezones for example here, in column TZ.

To start Chrome, use these commands:

• for Mac OS X: TZ='US/Pacific' open -na "Google Chrome" --args "--user-data-dir=$HOME/chrome-profile"
• for Linux: TZ='US/Pacific' google-chrome "--user-data-dir=$HOME/chrome-profile"

You don't need to change OS variables or extensions to do this in 2021.

Chrome Dev Tools > Hamburger Menu > More Tools > Sensors

On the Location section select your location or give it a timezone id (e.g. America/Chicago, America/New_York)

Unfortunately, JavaScript is only aware of the current time zone, as it is set by the operating system. There are no facilities to let the Date object use a different time zone in a particular context. There are some libraries to do time zone conversions, but that won't help for what you are asking.

On Linux, Mac OSX, and other *NIX systems, you can set the TZ environment variable. See Benedikt Köppel's answer for details.

However, there is no facility for this on Windows. Some Windows programs may pick up on the TZ environment variable, but those that do will not necessarily interpret it correctly, as they aren't wired up to support IANA time zone names. More on this here and here.