During the installation Calamares tries to make contact with a server to know your timezone.

Calamares checks your timezone with this code (February 2019).

Calamares tries to make contact with a specific url

https://geoip.kde.org/v1/calamares

But sometimes the service is offline (maintenance or other reasons).

You can interrupt this automatic search and set the timezone yourself with this easy trick.

Disable the networking via the network icon in system panel.

Set your timezone manually.

Enable the networking again via the network icon in system panel.