Much harder without consumable sample data, but something like below should be really close at least.  I'm not 100% sure on the specific end date determination you want, but you can adjust that as needed.

You don't really need a calendar table, an in-line tally table will do just fine and is much less overhead.


DECLARE @start_date date
DECLARE @end_date date

SET @start_date = DATEADD(YEAR, -1, GETDATE())
SET @end_date = GETDATE()

;WITH
cteTally10 AS (
  SELECT * FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) AS numbers(number)
),
cteTally1000 AS (
  SELECT ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) - 1 AS number
  FROM cteTally10 c1
  CROSS JOIN cteTally10 c2
  CROSS JOIN cteTally10 c3
)
SELECT

  CONVERT(char(4), date, 112) AS Year,

  CONVERT(char(2), date, 1) AS Month,

  CONVERT(char(2), date, 3) AS Day,

  ActiveOrdersCount

FROM (

  SELECT

    date, COUNT(*) AS ActiveOrdersCount

  FROM cteTally1000 days

  CROSS APPLY (
   SELECT DATEADD(DAY, days.number, @start_date) AS date
  ) AS calc_next_date

  INNER JOIN dbo.OrderTable ot ON ot.StartDtm >= date AND
   (ot.StopDtm IS NULL OR ot.StopDtm > date)
 
  WHERE days.number BETWEEN 0 AND DATEDIFF(DAY, @start_date, @end_date) - 1

) AS derived