Cannot convert from UTF8 to ASCII

I used some online encoding detection so all of them are saying that

2015%2d03%2d31

is encoded by UTF8.

So I used this code to decode it and to see

2015-03-31

But it doesn't work.

private static string UTF8toASCII(string text)
        {
            System.Text.Encoding utf8 = System.Text.Encoding.UTF8;
            Byte[] encodedBytes = utf8.GetBytes(text);
            Byte[] convertedBytes =
                    Encoding.Convert(Encoding.UTF8, Encoding.ASCII, encodedBytes);
            System.Text.Encoding ascii = System.Text.Encoding.ASCII;

            return ascii.GetString(convertedBytes);
        }

Any clue?