Why does a WHERE clause with an indexed, computed datetime column on an indexed view seek the clustered index on the original table
SQL Server expands the view and considers accessing the base tables instead. If the view or the base tables are used is a cost based decision by the optimizer. 

To force SQL Server to use your indexed view you should use the noexpand hint.

SELECT * 
from [AggregatedDateView] WITH(NOEXPAND) 
where [DateColumn] &gt; '2016-05-20' AND 
      [DateColumn] &lt; '2016-08-20'