wrong data of casestudy 2 in Operation analytics and investigation metric spike

In casestudy 2 of operation analytics and investigation metric , the users table data is incorrectly arranged in created_at column the pattern in date/month/year format is wrong somewhere they are arranged or separated by '/' ex-DD/MM/YY . and somewhere this is arranged in format DD-MM-YY . we are facing difficulty while running the sql query . please taaake a look into that