Obtain event time from JSON field.

[michael-bartmann]

Often the time a device reports some event over MQTT, the delay is not very big. Thus the event time (inside the message JSON) and the time of the MQTT message do not differ very much.
But specifically in case of retained messages the difference could be arbitrary long.
I have to admit that this feature is not trivial, as the JSON field containing the time needs to be configured, mapped and parsed.

[Edit: of course this is mostly relevant for charts]