How can I create a query or SP to correct SQL Date format issue

Hi I have some dates stored as Varchar within an SQL database, because of code issues in the application it is not possible to change the SQL fields to DateTime, I can report on the data by date if I Convert the string to 103 UK format, but I have many users who are US based is there anyway to check the local/format of the fields then have sql return the correct data by either a view or SP  as I do not really want to have seperate reports due to Date issues?

US systems report the error attached.

SELECT
  Table1.AD_Vendor,
  Table1.AD_Owner,
  Table1.AD_ContractPhone,
  Table1.AD_AgreementNumber,
  Table1.AD_ContractNumber,
  Table1.AD_ContractStartDate,
  Table1.AD_ContractEndDate
   FROM
  Table1
WHERE
  CONVERT(datetime, AD_ContractEndDate, 103) < DATEADD(MONTH, 3, GETDATE())
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