MQTT set (write) with micro-ebusd time and date

[brecak]

Hi,

My heating controller (Vaillant auroMATIC 620) is from an older generation that lacks automatic Daylight Saving Time (DST) adjustment.

I am trying to automatically update the date and time twice a year via MQTT. However, I can't get it to work.

When I publish a message to ebusd/broadcast/Vdatetime/set with the payload:
{"time":"23:31:02","date":"28.09.2026","$qq":16}

I successfully receive a confirmation MQTT response with {"time":"23:31:02","date":"28.09.2026","$qq":49}, but the time on the controller does not actually update. It stays unchanged, and the controller continues publishing its old broadcast messages (e.g. {"time":"23:28:32","date":"28.09.2026","$qq":16}).

I have a valid micro-ebusd token write permission enabled.

I also tried updating it via the web interface under Messages (where circuit broadcast and name Vdatetime are marked as w, u), filling in time: 23:31:02 (BTI) and date: 28.09.2026 (BDA), but the time still doesn't change on the device.

Is Vdatetime a read-only broadcast on the auroMATIC 620, or is there a specific target circuit/command required to set the system clock on this controller?

Thank you for your guidance!