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
* @param cdt2
	 *            The second datetime value
	 * @param iUnit
	 *            The field with respect to which the difference is being
	 *            computed as a fraction
	 * @param trimBelow
	 *            Specifies if trim the unit below the given unit.
	 * 
	 * @return The fractional difference between the two specified datetime
	 *         values
	 */
	public static final double computeDifference( CDateTime cdt1,
			CDateTime cdt2, int iUnit, boolean trimBelow )
	{
		final long l1 = cdt1.getTimeInMillis( );
		final long l2 = cdt2.getTimeInMillis( );
		if ( iUnit == Calendar.MILLISECOND )
		{
			double rt = ( l1 - l2 );
			if ( trimBelow )
			{
				rt = Math.floor( rt );
			}
			return rt;
		}
		else if ( iUnit == Calendar.SECOND )
		{
			double rt = ( l1 - l2 ) / MILLIS_IN_SECOND;
			if ( trimBelow )
			{