Hello Proluxon,
At the moment there is no DNS client implemented inside the CueCore2. This is why it is not possible to use a URL or domain name. At the moment there are no plans to add this functionality.
There are third party servers available for example google offers a couple of time servers. And there are multiple other third party servers available. For the best result you will have to find a server which is close to your installation.
We do not have a list of time servers which are a "good" option everywhere. If you can not find a time server, you could consider setting up you own time server. This allows you to get a time server which you control yourself.
Best regards,
Guido