Full disclosure, I attempted to convert from this function I wrote 10 years ago, but am getting the math wrong now.  - I'm pretty sure each part in this script works, I'm just not applying the new variables correctly.
CREATE FUNCTION [dbo].[udfConvertFromGMT]
(
  @InputDate DATETIME
)
RETURNS DATETIME
AS
/*
Script Date: 08/11/2009
Author: Sean Senneka
Purpose: Converts from GMT to CST (or CDT if date is during daylight savings time)
*/
--DECLARE @InputDate datetime
--Select @InputDate = '10/4/1985 08:00:00'
BEGIN
  DECLARE @converted_date DATETIME;
  DECLARE @DST_Start DATETIME;
  DECLARE @DST_End DATETIME;
  DECLARE @GMT_Offset_Destination INT;
  -- Get start and end dates for daylight savingstime for the year in question
  SELECT @DST_Start = CASE
          WHEN DATEPART(YEAR, @InputDate) >= 2007 THEN
           '3/1/' + CONVERT(VARCHAR, DATEPART(YEAR, @InputDate))
          WHEN DATEPART(YEAR, @InputDate)
           BETWEEN 1986 AND 2006 THEN
           '4/1/' + CONVERT(VARCHAR, DATEPART(YEAR, @InputDate))
          ELSE -- Year prior to 1986

           '4/30/' + CONVERT(VARCHAR, DATEPART(YEAR, @InputDate))
        END,
    @DST_End = CASE
         WHEN DATEPART(YEAR, @InputDate) >= 2007 THEN
          '11/1/' + CONVERT(VARCHAR, DATEPART(YEAR, @InputDate))
         ELSE
          '10/31/' + CONVERT(VARCHAR, DATEPART(YEAR, @InputDate))
        END;

  SELECT @DST_Start
   =
   --2007 till present get second Sunday of March, 1986-2006 get first Sunday of April, prior to 1986 get last sunday in April

   CASE
    WHEN DATEPART(YEAR, @InputDate) >= 2007 THEN
      DATEADD(
         HOUR,
         8,
         DATEADD(
             DAY,
             ((CEILING((CAST(17 AS DECIMAL(8, 2)) % DATEPART(dw, @DST_Start)) / 10) * 7) + 8)
             - DATEPART(dw, @DST_Start),
             @DST_Start
            )
        )
    WHEN DATEPART(YEAR, @InputDate)
      BETWEEN 1986 AND 2006 THEN
      DATEADD(
         HOUR,
         8,
         DATEADD(
             DAY,
             ((CEILING((CAST(17 AS DECIMAL(8, 2)) % DATEPART(dw, @DST_Start)) / 10) * 7) + 1)
             - DATEPART(dw, @DST_Start),
             @DST_Start
            )
        )
    ELSE -- Year prior to 1986

      DATEADD(HOUR, 8, DATEADD(DAY, 0 - (DATEPART(dw, @DST_Start) - 1), @DST_Start))
   END,

    --2007 till present get first Sunday of November, else get last Sunday of October

    @DST_End
     = CASE
       WHEN DATEPART(YEAR, @InputDate) >= 2007 THEN
         DATEADD(
            SECOND,
            -1,
            DATEADD(
                HOUR,
                8,
                DATEADD(
                    DAY,
                    ((CEILING((CAST(17 AS DECIMAL(8, 2)) % DATEPART(dw, @DST_End))
                       / 10
                       ) * 7
                    ) + 1
                    ) - DATEPART(dw, @DST_End),
                    @DST_End
                  )
               )
           )
       ELSE
         DATEADD(SECOND, -1, DATEADD(HOUR, 8, DATEADD(DAY, 0 - (DATEPART(dw, @DST_End) - 1), @DST_End)))
      END,
    @GMT_Offset_Destination = -360;

  RETURN CASE
     WHEN @InputDate
       BETWEEN @DST_Start AND @DST_End THEN
       DATEADD(MINUTE, @GMT_Offset_Destination + 60, @InputDate)
     ELSE
       DATEADD(MINUTE, @GMT_Offset_Destination, @InputDate)
    END;

END;