Bug 197589 Need constant for automaic attribute name "DateTime"
Currently when set dateTimeLevelType for cube level, model will automatically create a built-in attribute "DateTime". But the name is hard-coded. We need a public constant for this name to allow UI to access it.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
catch ( PropertyValueException ex )
		{
			ex.setElement( element );
			ex.setPropertyName( prop.getName( ) );
			throw ex;
		}
		if ( !( retValue instanceof ElementRefValue ) )
			return retValue;
		
		
		ElementRefValue refValue = (ElementRefValue) retValue;
		if ( refValue.isResolved( )
				&& value instanceof DesignElementHandle
				&& refValue.getElement( ) != ( (DesignElementHandle) value )
						.getElement( ) )
			throw new SemanticError( element, new String[]{prop.getName( ),
					refValue.getName( )},
					SemanticError.DESIGN_EXCEPTION_INVALID_ELEMENT_REF );
		return retValue;
	}
	/**
	 * Sets the value of an intrinsic property.
	 * 
	 * @param prop
	 *            definition of the property to set
	 * @param value
	 *            the property value to set.
	 * @throws SemanticException
	 *             if failed to set property.
	 */