Setting a different or own time server is possible with firmware 1.5.0 .
The setting can not (yet) be found in the app, but only in the WEB UI (calling the IP in the browser).
Under "Internet & Security" you will find the submenu "SNTP-SERVER".
There you can enter the IP address of the desired time server in the format xxx.xxx.xxx.xxx and save it with the button "SAVE".
The change is accepted after a reboot of the device.
In firmware 1.5.0, the setting only works if Internet access is not generally blocked for the Shelly device!