Konwertuj dzień roku na datetime w SQL Server

Pobierz część roku, przekonwertuj na datetime + dodaj dni

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

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

Lub:

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

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

Lub jeśli yyyyxxx to INT:

DECLARE @var INT = 2015223;

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

LiveDemo