A check for Das Erste
Check the programme on the Das Erste listing rather than a regional ARD stream with a similar name. Use the programme's date and channel label to make the guide comparison reproducible.
Check the station edition
Record one programme and its scheduled local time on the broadcaster page. Compare it with the channel label and time displayed in your player.
Check the guide if the live feed works
If the programme matches the live edition but the guide is wrong, confirm the channel mapping before changing the device clock.