Jeg har kunder i forskellige tidszoner

If you have clients in different time zones, set Show booking page in client timezone and Send notifications in client timezone in Settings -> Main configuration to Yes, so that your clients see booking page in their time zone.

For example: You are in UK, London and your client is in US, Washington. Your opening hours are 9a.m-5p.m GMT, but for your American client the schedule will be 2a.m-10a.m PDT.

If all your clients are in the same time zone, just set NO in both fields.