Assigning avg to columns and grouping that info by date

DECLARE @TEST1 TABLE ( ACCT INT NOT NULL, D_TIME DATE NULL, TOKEN INT NULL)
INSERT INTO @TEST1 VALUES (1,'01/01/2015', 5),(1,'02/15/2015', 6) ,(2,'02/18/2015', 4) ,(3,'01/11/2015', 3) ,(2,'03/30/2015', 1) ,(3,'03/01/2015', 4) ,(4,'03/01/2015', 1) ,(5,'02/21/2015', 3) ,(6,'02/28/2015', 2) 
DECLARE  @Start_DT DATE, @End_DT DATE
SET @Start_DT = '01/01/2015'
SET @End_DT = '03/23/2015'

SELECT COUNT(ACCT) AS count, DATEADD(m,DATEDIFF(m,0,D_TIME),0) AS monthStart, CONVERT(DECIMAL(10,2),(AVG(CONVERT(DECIMAL(10,2), NULLIF(TOKEN,0))))) AS avgToken
  FROM @TEST1
WHERE (D_TIME BETWEEN @Start_DT AND @End_DT)
  AND D_TIME IS NOT null
GROUP BY DATEADD(m,DATEDIFF(m,0,D_TIME),0)
ORDER BY  DATEADD(m,DATEDIFF(m,0,D_TIME),0) DESC

All I did here was remove your parameters, and alias your columns.

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