In that case use Eirikur's trigger although you could simplify it to
CREATE TRIGGER trg_LastSaleDate ON dbo.Transheaders
AFTER INSERT, UPDATE
AS
IF UPDATE(TradingDate)
BEGIN
UPDATE c
SET c.ZLastDate = i.TradingDate
FROM inserted i
JOIN dbo.Customers c ON c.UniqueID = i.UniqueID
END
Far away is close at hand in the images of elsewhere.
Anon.