How to Convert Epoch Time to Eastern Time in MS SQL

USE [DATABASE]
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:        Leonard Nelson
-- Create date: 01/11/2011
-- Description:    Convert Eastern Time to Epoch Time. 
-- Definition: Epoch time is the number of seconds that have elapsed since January 1, 1970 (midnight UTC/GMT)
-- Syntax: SELECT dbo.FunctionConvertToEpochTime('2011-01-19 16:27:15')
-- 1/19/2011 4:27:15 PM EST = 1295472435
-- =============================================
ALTER FUNCTION [dbo].[FunctionConvertToEpochTime]
(
    @InputLocalTime nvarchar(19)
)
RETURNS int
AS
BEGIN

-- Convert the Input Time Stamp to 24 hour format yyyy-mm-dd hh:mi:ss
SET @InputLocalTime = CONVERT(nvarchar, @InputLocalTime, 120);

DECLARE @EpochTime int, @UTCTime datetime, @CurrentTimeZoneOffset int, @TimeZoneOffset int;

-- Set the Time Zone Offset
SET @CurrentTimeZoneOffset = DATEDIFF(hh, GETUTCDATE(), GETDATE());

-- Set the Eastern Time Zone and factor for Daylight Savings Time Offset
SET @TimeZoneOffset =
    CASE 
        -- The schedule through 2006 in the United States was that DST began on the first Sunday in April 
        -- (April 2, 2006), and changed back to standard time on the last Sunday in October (October 29, 2006). 
        -- The time is adjusted at 02:00 local time.    
        WHEN YEAR(@InputLocalTime) < = 2006 THEN
            CASE 
                WHEN 
                      CAST(@InputLocalTime AS datetime) >=  '4/' + CAST(ABS(8-DATEPART(dw,'4/1/' + CAST(YEAR(@InputLocalTime) as varchar)))%7 + 1 as varchar) +  '/' + CAST(YEAR(@InputLocalTime) as varchar) + ' 2:00' 
                  AND 
                      CAST(@InputLocalTime AS datetime) < '10/' + CAST(32-DATEPART(dw,'10/31/' + CAST(YEAR(@InputLocalTime) as varchar)) as varchar) +  '/' + CAST(YEAR(@InputLocalTime) as varchar) + ' 2:00' 
                THEN 
                    -- Time Zone EST DST Offset should be 4
                    -4
                ELSE
                    -- Time Zone EST DST Offset should be 5
                    -5
            END
        ELSE
            CASE
            -- By the Energy Policy Act of 2005, daylight saving time (DST) was extended in the United States in 2007. 
            -- DST starts on the second Sunday of March, which is three weeks earlier than in the past, and it ends on 
            -- the first Sunday of November, one week later than in years past. This change resulted in a new DST period 
            -- that is four weeks (five in years when March has five Sundays) longer than in previous years.[35] In 2008 
            -- daylight saving time ended at 02:00 on Sunday, November 2, and in 2009 it began at 02:00 on Sunday, March 8.[36]
                WHEN
                      CAST(@InputLocalTime AS datetime) >= '3/' + CAST(ABS(8-DATEPART(dw,'3/1/' + CAST(YEAR(@InputLocalTime) as varchar)))%7 + 8 as varchar) +  '/' + CAST(YEAR(@InputLocalTime) as varchar) + ' 2:00' 
                  AND 
                      CAST(@InputLocalTime AS datetime)  < '11/' + CAST(ABS(8-DATEPART(dw,'11/1/' + CAST(YEAR(@InputLocalTime) as varchar)))%7 + 1 as varchar) +  '/' + CAST(YEAR(@InputLocalTime) as varchar) + ' 2:00'
                THEN 
                    -- Time Zone EST DST Offset should be 4
                    -4
                ELSE
                    -- Time Zone EST DST Offset should be 5
                    -5
            END
        END

-- Set the TimeZoneDSTOffset 
IF @TimeZoneOffset > 0
    BEGIN
        SET @TimeZoneOffset = -ABS(@TimeZoneOffset);
    END
ELSE
    BEGIN
        SET @TimeZoneOffset = ABS(@TimeZoneOffset);
    END

-- Add the Time Zone DST Offset to the Input Time Stamp to calculate the UTC Time
SET @UTCTime = DATEADD(hour, @TimeZoneOffset, @InputLocalTime);

-- Subtract the number of seconds to calculate Epoch Time
SET @EpochTime = DATEDIFF(s, '1970-01-01 00:00:00', @UTCTime);

-- Return the EpochTime
RETURN (@EpochTime);
END