年の日を SQL Server の datetime に変換する

年の部分を取得し、日時に変換 + 日数を追加

DECLARE @var NVARCHAR(100) = '2015223';

SELECT CAST(LEFT(@var, 4) AS DATETIME) +  CAST(RIGHT(@var, 3) AS INT) - 1;

または:

DECLARE @var NVARCHAR(100) = '2015223';

SELECT DATEADD(dd,CAST(RIGHT(@var, 3) AS INT)-1, CAST(LEFT(@var, 4) AS DATETIME))

または、yyyyxxx が INT の場合:

DECLARE @var INT = 2015223;

SELECT DATEADD(dd,(@var%1000)-1, CAST(CAST((@var/1000) AS NVARCHAR(100)) 
               AS DATETIME));

LiveDemo