2020 update: see the other answer, which shows the result of a feature request I filed with Chromium to implement a way to override the timezone.
Note that the technique only works to change the location (including timezone) in that tab. Other tabs will still use the system's timezone. DevTools isn't a reliable way to spoof your timezone.
To change the timezone for all tabs in the browser (Chrome or Firefox or whatever), on Linux/MacOS, you can launch the browser with the TZ environment variable set to the desired timezone name:
To do this in Chrome Dev tools you can use the Geolocation sensor emulation.
Go to the 3 dotted menu, More Tools and Sensors.
There's a Geolocation dropdown with some common locations but you can change your preferred one as well. To do that you can choose Other... in that dropdown and enter a custom Timezone ID
You can also do this with the Vytal extension which uses the devtool location sensor emulation. It can spoof your timezone, locale, geolocation and user agent. You can also automatically set your location data to match your ip address.