Hi Lulas,
The top half of this diagram might help
start_timeis the startTime of the whole compositionoriginis the start time of a whole Observationtime(Event Time) is the startTime of an single Event within an Observation, where there potentially multiple events
In a simple use-case e.g a simple Vital signs composition, all 3 dateTimes may well be identical, and in fact some CDRs allow you to default the origin and Event.time to start_time if not supplied explicitly.
However there are situaitons where the 3 times may differ, and be important to record explicitly.
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Where the time that the Observation was recorded needs to be precise, and may be different from the overall composition.context.start_time e.g in a Lab report
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Where an Observation is made multiple times e.g an Apgar score at various intervals and each needs to be timed explicitly, using multiple events.