<property name="activeFrom">
   <column name="ActiveFrom"/>
   <type name="no.bekk.mobiletimekeeper.customtypes.CalendarStringType"/>
</property>

<composite-id>
   <key-many-to-one name="project" class="Project" column="ProjectID"/>
   <key-many-to-one name="employee" class="Employee" column="EmployeeID"/>
   <key-property name="date"
      column="ActualDate"
      type="no.bekk.mobiletimekeeper.CalendarStringType">
   </key-property>
</composite-id>

public class CalendarStringType implements UserType
{
  private static final int[] SQL_TYPES = {Types.VARCHAR};
 
  public CalendarStringType() {}
 
  public int[] sqlTypes()
  {
    return SQL_TYPES;
  }

  /**
   * Tells Hibernate what Java type is mapped by this UserType.
   * @return The Java type which is mapped by this UserType   
   */
  public Class returnedClass()
  {
    return GregorianCalendar.class;
  }

  /**
   * The UserType is responsible for dirty-checking property values. The equals()
   * method compares the current property value to a previous snapshot and determines
   * whether the property is dirty and must by saved to the database.
   */
  public boolean equals( Object x, Object y ) throws HibernateException
  {
    if( x != null && y != null)
      return ((GregorianCalendar)x).equals( (GregorianCalendar)y );
    return false;
  }

  /**
   * @return the objects hashcode
   */
  public int hashCode( Object x ) throws HibernateException
  {
    return ((GregorianCalendar)x).hashCode();
  }

  /**
   * retrieves the property value from the JDBC ResultSet.
   */
  public Object nullSafeGet( ResultSet resultSet, String[] names, Object owner )
     throws HibernateException, SQLException
  {
    if (resultSet.wasNull())
      return null;
   
    //The true/false value is stored in the database as tinyint.
    String dateString = resultSet.getString( names[0] );
   
    //returning a new Calendar object whoose value is set according to the value
    //of the String date. In short: parsing the retrieved string and makes a new
    //Calendar objects
    return parseDateString( dateString );
   
  }

  /**
   * writes the property value to the JDBC PreparedStatement
   */
  public void nullSafeSet( PreparedStatement prepStatement, Object value, int index )
     throws HibernateException, SQLException
  {
    if (value == null)
    {
      prepStatement.setNull( index, Types.VARCHAR );
    }
    else
    {
      String year = ((GregorianCalendar)value).get( Calendar.YEAR ) + "";
      String month = Toolbox.twoDigits( ((GregorianCalendar)value).get( Calendar.MONTH ) );
      String date = Toolbox.twoDigits( ((GregorianCalendar)value).get( Calendar.DATE ) );
     
      //perform a conversion to get Hibernate to store an appropriate VARCHAR value
      //from the String object.
      prepStatement.setString( index, year + month + date );
     
    }
  }

  /* (non-Javadoc)
   * @see org.hibernate.usertype.UserType#deepCopy(java.lang.Object)
   */
  public Object deepCopy( Object value ) throws HibernateException
  {
    //extract the date parameters
    int year = ((GregorianCalendar)value).get( Calendar.YEAR );
    int month = ((GregorianCalendar)value).get( Calendar.MONTH );
    int date = ((GregorianCalendar)value).get( Calendar.DATE );
   
    return new GregorianCalendar( year, month, date );
  }

  /**
   * The Java type Calendar is mutable (the type's contents can be changed after instanciation),
   * hence this method will return true.
   * @return will always return true.
   */
  public boolean isMutable()
  {
    return true;
  }

  /* (non-Javadoc)
   * @see org.hibernate.usertype.UserType#disassemble(java.lang.Object)
   */
  public Serializable disassemble( Object value ) throws HibernateException
  {
    return null;
  }

  /* (non-Javadoc)
   * @see org.hibernate.usertype.UserType#assemble(java.io.Serializable, java.lang.Object)
   */
  public Object assemble( Serializable cached, Object owner )
                                                             throws HibernateException
  {
    return null;
  }

  /* (non-Javadoc)
   * @see org.hibernate.usertype.UserType#replace(java.lang.Object, java.lang.Object, java.lang.Object)
   */
  public Object replace( Object original, Object target, Object owner )
                                                                       throws HibernateException
  {
    return null;
  }
 
  /**
   * A helper method to do the parsing from a YYYYMMDD-formatted String
   * to a Java Calendar object.
   * @param toBeParsed The String to parse. It must be in the format YYYYMMDD
   * @param calendar the Calendar object that shall recieve the date parameters from the parsed
   *                 String
   * @throws NumberFormatException If the String toBeParsed isn't correctly formatted, this
   *                               exception is thrown.
   */
  private Calendar parseDateString( String toBeParsed ) throws NumberFormatException
  {
    //instanciate a new Calendar object
    GregorianCalendar calendar = new GregorianCalendar();
     
    if( toBeParsed.length() == 8 )
    {
      //extract the year, month and day from the recieved string
      int year = Integer.parseInt( toBeParsed.substring(0,4) );
      int month = Integer.parseInt( toBeParsed.substring(4,6) ) - 1; //subtract 1, months are from 0 to 11
      int day = Integer.parseInt( toBeParsed.substring(6,8) );

      //store the date
      calendar.set(year, month, day);
      return calendar;
    }
    else
    {
      throw new NumberFormatException("The date string must have the exact " +
        "length of 8. It must also only contain digits.\nDebug info:\n" +
        "String toBeParsed = " + toBeParsed );
    }
  }

}

<key-property name="date" column="ActualDate"/>