Non Clustered Index to help Clustered Index ? (Corrected!)

CREATE TABLE [dbo].[TimeValues]
(
    [Time] DATETIME NOT NULL,
    [Collection_Id] INT NOT NULL,
    [Value] DECIMAL(18,6) NOT NULL,
    CONSTRAINT [PK_TimeValues] PRIMARY KEY ([Time],[Collection_Id]) ON [MyPartitionScheme]([Time])
) ON [MyPartitionScheme]([Time])

--The following script takes 2 min to run ???
SELECT
    DATEADD(DAY,DATEDIFF(DAY,'2000-01-01 00:00:00',[Time]),'2000-01-01 00:00:00') AS [Time]
    ,1 AS [Collection_Id]
    ,SUM([Value]) AS [Value]
FROM [dbo].[TimeValues]
WHERE [Time] BETWEEN '2000-01-01' AND '2020-01-01'
    AND Collection_Id = 1
GROUP BY DATEDIFF(DAY,'2000-01-01 00:00:00',[Time])

--If I add the following Index the script takes 0 sec. to run WHY ????
CREATE NONCLUSTERED INDEX [IX_TimeValues_Collection_Id_Time]
ON [dbo].[TimeValues]([Collection_Id],[Time])
INCLUDE ([Value]) WITH (FILLFACTOR = 80)