I was also facing the same issue where I was receiving the Transaction_Date as YYYYMMDD in bigint format. So I converted it into Datetime format using below query and saved it in new column with datetime format. I hope this will help you as well.
SELECT
convert( Datetime, STUFF(STUFF(Transaction_Date, 5, 0, '-'), 8, 0, '-'), 120) As [Transaction_Date_New]
FROM mydb