In a web application, it’s a common pattern to have the backend to send a UTC timestamp, which the frontend will later display in the user’s timezone. In these situations, you may want to spoof your browser’s location in order to verify that things are working as you’d expect.

This is possible in Chromium and in Google Chrome, by opening the developer tools (F12), and pull up the menu with three dots ⋮ (next to the developer tools close button), then choose More Tools -> Sensors, and you’ll see options to override geolocation sensors.

An image is worth a thousand words:

The console displays the result of running new Date(), before and after overriding the location.

In Firefox

Unfortunately, there is no graphical user interface to do the same in Firefox. 😢

You’re forced to fiddle with the about:config settings, the method is explained here

DISCLAIMER: I have not tried it!