Why use all those substrings anyways?

declare @TestData table (

AS400Dates VARCHAR(10)

);

insert into @TestData

VALUES

('0991231'),

('0991015'),

('0970704'),

('1080518'),

('1080707'),

('1080515'),

('1080731'),

('1080815'),

('1080822'),

('1080911'),

('1080916'),

('1080925'),

('1080926'),

('1080927'),

('1081023');

SELECT

AS400Dates,

CAST(AS400Dates AS INT) DateAsInt,

19000000 + CAST(AS400Dates AS INT) DateAsInt2,

CAST(CAST(19000000 + CAST(AS400Dates AS INT) AS VARCHAR) AS DATE) DateAsDate

FROM

@TestData;