To reference an interval value in a dimension_group of type: duration, what syntax is required?

When referencing an interval value in a dimension_group of type duration in LookML, the correct syntax is to use ${interval_fieldname}. This is because the duration type is specifically designed to work with interval fields that represent spans of time. The dimension_group handles various aspects of time interval analysis, and referencing the interval field directly allows the Looker platform to correctly interpret and manipulate the duration data for calculating metrics or creating visualizations.

In contrast, using syntax like ${duration_fieldname}, ${time_fieldname}, or ${date_fieldname} does not specifically communicate the intention to access interval values, which are crucial for accurate calculations related to durations. Each of those alternatives serves different purposes within LookML and does not align with the expected syntax for duration intervals.