Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/util/CDateTime.java
*         occurred
	 */
	public static final int getDifference( CDateTime cdt1, CDateTime cdt2 )
	{
		if ( cdt1.getYear( ) != cdt2.getYear( ) )
		{
			return Calendar.YEAR;
		}
		else if ( cdt1.getMonth( ) != cdt2.getMonth( ) )
		{
			return Calendar.MONTH;
		}
		else if ( cdt1.getDay( ) != cdt2.getDay( ) )
		{
			return Calendar.DATE;
		}
		else if ( cdt1.getHour( ) != cdt2.getHour( ) )
		{
			return Calendar.HOUR_OF_DAY;
		}
		else if ( cdt1.getMinute( ) != cdt2.getMinute( ) )
		{
			return Calendar.MINUTE;
		}
		else if ( cdt1.getSecond( ) != cdt2.getSecond( ) )
		{
			return Calendar.SECOND;
		}
		else
		{