Just the time part? OK, take the fractional part only:

DECLARE @test-2 TABLE (

dtColumn datetime

)

INSERT INTO @test-2 VALUES (GETDATE() + RAND())

INSERT INTO @test-2 VALUES (GETDATE() + RAND())

INSERT INTO @test-2 VALUES (GETDATE() + RAND())

INSERT INTO @test-2 VALUES (GETDATE() + RAND())

SELECT CONVERT(char(8),CAST(AVG(CAST(dtColumn AS FLOAT) - FLOOR(CAST(dtColumn AS FLOAT))) AS datetime),108)

FROM @test-2