I don't agree. I can't think of one example where the server is responsible for knowing the client's time zone. I know in this case the sensors are all in my house and therefore in the same timezone.
I'm using Home Assistant but AWI mentioned that "I had similar issues with Domoticz". This was because the server is not responsible for knowing which of the 192 possible DST & time zone combinations applies to the client.
Further to this assertion, the code I'm using is the MySensors code from the page I linked to above. This means that the sensor is expecting epoch time. Recoding the servers to send the dates in ISO 8601 format doesn't fix anything if the client is in a different time zone - the client would still need to adjust the date to be in the correct time zone for its physical location. Again, I know that in my specific use case the sensors and the controller are in the same location, I'm just trying to clarify that the controller does not have responsibility for sending the correct time to client - that's not how the whole internet works.
Also, Epoch time isn't a "format". It's a linear counter that increases with time ad infinitum. In other words, it's just a number. 1468069791 is just a number - the question is, is it epoch time, an account number, the distance from here to Jupiter?
Anyway - I found this web page which answered my question.