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
rt = Math.floor( rt );
			}
			return rt;
		}
		else if ( iUnit == Calendar.MINUTE )
		{
			double rt = ( l1 - l2 ) / MILLIS_IN_MINUTE;
			if ( trimBelow )
			{
				rt = Math.floor( rt );
			}
			return rt;
		}
		else if ( iUnit == Calendar.HOUR_OF_DAY )
		{
			double rt = ( l1 - l2 ) / MILLIS_IN_HOUR;
			if ( trimBelow )
			{
				rt = Math.floor( rt );
			}
			return rt;
		}
		else if ( iUnit == Calendar.DATE )
		{
			double rt = ( l1 - l2 ) / MILLIS_IN_DAY;
			if ( trimBelow )
			{
				rt = Math.floor( rt );
			}
			return rt;