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Timeframes, fixed points in time, and other milestones may be defined in the context of a study as references to which the timing of other collected observations is related. In such cases, timeframes are referred to as reference periods and such fixed points in time and milestones are referred to as reference time points.

The start and end dates (and times when applicable) of a reference period in a study are represented using variables RFSTDTC and RFENDTC respectively in the Demograph

In general observation class domains, days relative to the reference start date or in relation to applicant-defined reference dates may be represented for observations.

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