MIN, DATEADD, DATEPART, DATEDIFF functions cause table scan
Try this approach instead (assuming the point of all the dateadd/datediff operations are just to set the millisecond component to 0). The query was adapted to the AdventureWorks database:

SELECT 
  CONVERT(DATETIME, CONVERT(CHAR(19), MIN(OrderDate), 121)), 
  CONVERT(DATETIME, CONVERT(CHAR(19), MAX(OrderDate), 121))
FROM Sales.SalesOrderHeader GROUP BY ();


The trick is to find the rows before you start messing with the column values applying all kinds of formatting and date manipulation. This results in a much more efficient plan as long as there is a supporting index on the OrderDate column (and no worse than your current query if there is no supporting index):


  


Yes, they are still scans, and there are two instead of one. But the impact on actual execution is undeniable: