首先用 yyyy 更改年格式化程序，而不是使用两个 NSDateFormatter 只使用这样一个

First changes your year formatter with yyyy and instead of using two NSDateFormatter use just one like this

let inputFormatter = NSDateFormatter()
inputFormatter.dateFormat = "MM/dd/yyyy"
let showDate = inputFormatter.dateFromString("07/21/2016")
inputFormatter.dateFormat = "yyyy-MM-dd"
let resultString = inputFormatter.stringFromDate(showDate!)
print(resultString)

对于swift3

func convertDateFormater(_ date: String) -> String
    {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss z"
        let date = dateFormatter.date(from: date)
        dateFormatter.dateFormat = "yyyy-MM-dd"
        return  dateFormatter.string(from: date!)

    }