There’s a link in the warning message to the documentation
You have to add the following line line to your config.php:
'maintenance_window_start' => 1,
1 stands for the hour when you want the maintainenance window to start in UTC time, so in the above example the maintainenance window starts at 01:00 UTC Time.
Nextcloud Documentation:
https://docs.nextcloud.com/server/28/go.php?to=admin-background-jobs
What is UTC time?:
https://www.utctime.net/
UTC time zone converter:
https://www.utctime.net/utc-time-zone-converter