Get Date from ISO Week in SQL

Question

I created a function in SQL to return the proper date from a ISO Week from a format like "15W53". Where the first part before the "W" is the year number and the second half is the week number. The returned date should return the start of the week for that date.

So for example, 15W53 should return 12-28-2015 and 16W01 should return 01-04-2016. However if I run this for the following examples I get, from what I have read about ISO week, incorrect results.

Did I create the function incorrectly to parse the date?

SET DATEFIRST 1; 
SELECT dbo.[GetDateFromISOweek]('15W52') AS Correct, '15W52'  -- Returns: 2015-12-21
SELECT dbo.[GetDateFromISOweek]('15W53') AS Correct, '15W53'  -- Returns: 2015-12-28
SELECT dbo.[GetDateFromISOweek]('16W01') AS Incorrect, '16W01'-- Returns: 2015-12-28
SELECT dbo.[GetDateFromISOweek]('16W02') AS Incorrect, '16W02'-- Returns: 2016-01-04
SELECT dbo.[GetDateFromISOweek]('16W03') AS Incorrect, '16W03'-- Returns: 2016-01-11

Function:

CREATE FUNCTION [dbo].[GetDateFromISOweek] (@Input VARCHAR(10))  
RETURNS DATETIME  
WITH EXECUTE AS CALLER  
AS  
BEGIN  
    DECLARE @YearNum CHAR(4) 
    DECLARE @WeekNum VARCHAR(2)

    SET @YearNum = SUBSTRING(@Input,0,CHARINDEX('W',@Input,0))
    SET @WeekNum = SUBSTRING(@Input,CHARINDEX('W',@Input,0)+1,LEN(@Input))

    RETURN(DATEADD(wk, DATEDIFF(wk, 6, '1/1/' + @YearNum) + (@WeekNum-1), 7));
END; 

Answer 1

Change the -1 in your return to calculate if the first week of the year should be consider the first one or not (if have more than 3 days). Something like this

case when DATEDIFF ( day ,  convert(datetime,'01/01/'+ @YearNum),@FirstDay )>=3 then 1 else 0 end

The full code, including the first sunday, can be improved, but works...

CREATE FUNCTION [dbo].[GetDateFromISOweek] (@Input VARCHAR(10))  
RETURNS DATETIME  
WITH EXECUTE AS CALLER  
AS  
BEGIN  
    DECLARE @YearNum CHAR(4) 
    DECLARE @WeekNum VARCHAR(2)
    declare @FirstDay datetime

    SET @YearNum = cast(SUBSTRING(@Input,0,CHARINDEX('W',@Input,0)) as int)+2000
    SET @WeekNum = SUBSTRING(@Input,CHARINDEX('W',@Input,0)+1,LEN(@Input))
    set @FirstDay=DATEADD(DAY, (@@DATEFIRST - DATEPART(WEEKDAY, DATEADD(YEAR, @YearNum - 1900, 0)) +  (8 - @@DATEFIRST) * 2) % 7, DATEADD(YEAR, @YearNum - 1900, 0))-1

    RETURN(DATEADD(wk, DATEDIFF(wk, 6, '1/1/' + @YearNum) + (@WeekNum-case when DATEDIFF ( day ,  convert(datetime,'01/01/'+ @YearNum),@FirstDay )>=3 then 1 else 0 end), 7));
END; 
go
SET DATEFIRST 1; 
SELECT dbo.[GetDateFromISOweek]('15W52'),'15W52' union
SELECT dbo.[GetDateFromISOweek]('15W53'),'15W53' union
SELECT dbo.[GetDateFromISOweek]('16W01'), '16W01' union
SELECT dbo.[GetDateFromISOweek]('16W02'), '16W02' union
SELECT dbo.[GetDateFromISOweek]('16W03'), '16W03'

The result will be

----------------------- -----
2015-12-21 00:00:00.000 15W52
2015-12-28 00:00:00.000 15W53
2016-01-04 00:00:00.000 16W01
2016-01-11 00:00:00.000 16W02
2016-01-18 00:00:00.000 16W03