You could also try testing for specific formats
CREATE TABLE #dates (dateString varchar(10));
GO
INSERT INTO #dates
VALUES
('03/15/2017'),
('04/18/17'),
('14.06.15'),
('27.01.2014'),
('24th May 2'),
('09.09.2015'),
('42550'),
('27.07.16');

You have 2 updates to the same table, with a trigger firing on each update.

I would try doing a single update
UPDATE [dbo].[UO_MList]
SET GeneralSales_HolidaySales_OptIn...

This should do the trick.

SELECT NEWDATETIME = DATEADD(DD, DATEDIFF(DD, 0, DATEFIELD1), 0) + CAST(CAST(DATEFIELD2 AS TIME) AS DATETIME)