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Date hierarchies are regular drill down hierarchies, except they drill from coarse-grained to fine-grained temporal fields, in the correct order — Year— Year, Month, Day, for example. You can apply date functions to manufacture more coarse grained fields, such as the "month start date" date function to the Start Date field (which has daily day granularity), which would produce a new version of the Start Date field with monthly granularity.
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