new Datetime();错误？

[pody]

import java.text.SimpleDateFormat;
import java.util.Date;
import javax.swing.*;


public class No17 {

public class Datetime{

private Date today;
private SimpleDateFormat sdf;

Datetime(){
today =new Date();
sdf=new SimpleDateFormat("EEEE MMMM dd,yyyy");
}

public void showtime(){
JOptionPane.showMessageDialog(null, sdf.format(today));
}
}

public static void main(String[] args) {
Datetime today=new Datetime();
today.showtime();
}
}



显示错误：
java.lang.Error: Unresolved compilation problem:
Must explicitly qualify the allocation with an enclosing instance of type chapter2.No17 (e.g. x.new A() where x is an instance of chapter2.No17).

at java.lang.reflect.Constructor.newInstance(Native Method)
at chapter2.No17.main(No17.java:26)
Exception in thread "main"

[pody]

Datetime today=new Datetime();为什么会错误呢？如何修改？