Just a point of history. The "dynamic approach" to IS-LM is present in the work of Samuelson (1941), Modigliani (1944) and Klein (1947) in the form of dynamic systems based upon both differential and difference equations in the case of Samuelson (1941), that later became Ch IX of "Foundations", and in the dynamical model (difference equations) in Modigliani (1944: 62-64, eg. p. 63, equations 2.1-2.6). But, as Dave Colander correctly notes, it is difficult to "hang" things such as difference and differential euqations onto a two-dimensional notional diagram, and thus the textbook writers never bothered with the richer dynamic framework provided by Samuelson, Modigliani and Klein, until insightful authors such as George Horwich realized that something important was missing from the original Harrod-Meade-Hicks euqational-diagrammatic SILL framework.... Warren Young