Varchar m/d/yyyy到yyyy/mm/dd sql。

I have a file that inputs data in the format m/d/yyyy or m/dd/yyyy depending on whether or not the date value is 2 digits (example 4/1/2015 or 4/14/2015). I need to convert this to a date with the format yyyy/mm/dd (example 2015/04/01 or 2015/04/14). I have tried multiple way but I get the error "Invalid date" every time. Please help.I have a file that inputs data in the format m/