check 'PONumber' field
SUBSTRING(PONumber,6,2) is expressing
months not date
this should be return value between 1 to 12 only.
so,check results of SUBSTRING(PONumber,6,2) like given below
select
cast(SUBSTRING(PONumber,6,2) as int)
from dbo.tbl_PO
where cast(SUBSTRING(PONumber,6,2) as int)>12 or cast(SUBSTRING(PONumber,6,2) as int)=0
Happy coding!
:)