event_time is the timestamp that the Ardexa Agent picked up the given message and will always be UTC. event_time is present for all records in the Ardexa system.
On some devices monitored by the Ardexa Agent, instead of giving a raw reading when a request is made, they will provide a list of cached readings. This is where datetime is used. datetime is when the device being monitored took a reading, and event_time is when the Ardexa Agent picked up that reading for transmission to the cloud.
Therefore, where available, datetime is the actual time the measurement was taken.