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 Calendar.YEAR;
	}
	/**
	 * Computes the difference between two given datetime values as a fraction
	 * for the requested field.
	 * 
	 * @param cdt1
	 *            The first datetime value
	 * @param cdt2
	 *            The second datetime value
	 * @param iUnit
	 *            The field with respect to which the difference is being
	 *            computed as a fraction
	 * 
	 * @return The fractional difference between the two specified datetime
	 *         values
	 */
	public static final double computeDifference( CDateTime cdt1,
			CDateTime cdt2, int iUnit )
	{
		return computeDifference( cdt1, cdt2, iUnit, false );
	}
	/**
	 * Computes the difference between two given datetime values as a fraction
	 * for the requested field.
	 * 
	 * @param cdt1
	 *            The first datetime value