When the length of time an observation continues, i.e., the duration of the observation, is collected will be represented in applicable domains using variables:
--DTC or --STDTC for the start of the observation and --ENDTC for the end of the observation when date/times are collected.
--DUR when the duration of the observation is collected.
--DUR will be used when duration has been collected and start and end dates have not been collected.