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 null;
		}
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Object#toString()
	 */
	public final String toString( )
	{
		return _sdf.format( getTime( ) );
	}
	public boolean after( Object when )
	{
		if ( when == null )
		{
			return false;
		}
		return super.after( when );
	}
	public boolean before( Object when )
	{
		if ( when == null )
		{
			return false;
		}
		return super.before( when );
	}
}