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
* 
	 * @return The hour associated with this instance
	 */
	public final int getHour( )
	{
		return get( Calendar.HOUR_OF_DAY );
	}
	/**
	 * Returns the minute associated with this instance
	 * 
	 * @return The minute associated with this instance
	 */
	public final int getMinute( )
	{
		return get( Calendar.MINUTE );
	}
	/**
	 * Returns the second associated with this instance
	 * 
	 * @return The second associated with this instance
	 */
	public final int getSecond( )
	{
		return get( Calendar.SECOND );
	}
	/**
	 * Returns the most significant datetime unit in which there's a difference
	 * or 0 if there is no difference.
	 * 
	 * @return The least significant 'Calendar' unit in which a difference