FULL PRODUCT VERSION :
1.6.0_07-b06

ADDITIONAL OS VERSION INFORMATION :
WinXP SP2

A DESCRIPTION OF THE PROBLEM :
While Calendar objects are accepted by the compiler in the constructor of SpinnerDateModel as "Comparable", a ClassCastException is thrown at runtime.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No runtime error.
ACTUAL -
  Program aborted

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.util.Date cannot be cast to java.util.Calendar
        at java.util.Calendar.compareTo(Unknown Source)
        at javax.swing.SpinnerDateModel.<init>(Unknown Source)
        at JSpinnerDemo2.<init>(JSpinnerDemo2.java:19)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.*;
import java.util.*;
import javax.swing.*;

public class JSpinnerDemo extends JFrame {

  public JSpinnerDemo() {
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    Date ini= new Date(System.currentTimeMillis()-2592000000l); // -30 days
    Calendar start= Calendar.getInstance();
    start.set(1445, 0, 1);
    Calendar now= Calendar.getInstance();
    SpinnerDateModel spinModel= new SpinnerDateModel
(ini,start,now,Calendar.MONTH);
// ClassCastException: java.util.Date cannot be cast to java.util.Calendar

    JSpinner spinAcqDate = new JSpinner(spinModel);
    add(spinAcqDate);
    pack();
    setVisible(true);
  }

  public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
      public void run() {
new JSpinnerDemo();
      }
    });
  }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use Date instead of Calendar as parameters (Thanks to Camickr):

   SpinnerDateModel spinModel= new SpinnerDateModel
(ini,start.getTime(),now.getTime(),Calendar.MONTH);