DATADIFF (day, GETDATE(), Дата_начала_года)

USE MASTER
GO

DECLARE @InCurrentDate DATE;
DECLARE @InNewYearDate DATE;

SELECT @InCurrentDate=GETDATE();

SELECT @InNewYearDate=CAST(DATENAME(YEAR,@InCurrentDate)AS DATE);

SELECT @inCurrentDate AS CurrentDate,@InNewYearDate AS NewYearDate,
DATEDIFF(DD,@InNewYearDate,@InCurrentDate)AS DateDifff;

select datepart(dy, getdate());

--ваша таблица
WITH TAB AS
(
  SELECT 1 AS ID,CAST(GETDATE() AS DATE)AS DAteColumn
   UNION ALL
  SELECT 2 AS ID,'20170831'AS DateColumn
   UNION ALL
  SELECT 3 AS ID,'20050502'AS DateColumn
   UNION ALL
  SELECT 4 AS ID,'20121117'AS DateColumn   
)
SELECT T.ID,T.DateColumn,
CAST(DATENAME(YEAR,T.DateColumn)AS DATE)AS FirstYearDate,
DATEDIFF(DD,CAST(DATENAME(YEAR,T.DateColumn)AS DATE),T.DateColumn)AS DaysDifference
FROM TAB T;

DATEPART(DayOfYear, <ВашаДата(любая,вТомЧислеПолеТаблицы)>)

Datefromparts(Year,1,1)