The third book of On War has two chapters that interest us here, Sammlung der Kräfte im Raum and Vereinigung der Kräfte in der Zeit. I would render these “Assembly of forces in space” and “Unification of forces in time.” Note that the modern German would use Konzentration in the sense we use it ourselves to render the chapter titles Konzentration der Kräfte im Raum, etc. Clausewitz seems not only to be making a distinction between his two “concentrations” but also among concentration, assembly, and unification.
“Concentration in time” is a somewhat new English rendering, possibly early 20th Century, possibly via Jean Colin (though this Frenchman would have given the term its Gallic form not an English coinage.)
After the chapter on “Space,” what Clausewitz discusses in "Time" is mainly its comparative effects on force (esp. via attrition) at both the tactical and strategic levels. He labors through many words to bring forth three germane points:
… in Strategy we can never employ too many forces, and consequently also that they must be applied simultaneously to the immediate purpose …Clausewitz appears to have had in mind here the invasion of Russia by Napoleon, i.e. the application of all strategic forces to a single theatre, rather than the simultaneous application of force in multiple theatres.
… it cannot be an object to make time an ally on its own account by bringing troops [at the strategic level] successively into action …
… all forces which are available and destined for a strategic object should be simultaneously applied to it; and this application will be so much the more complete the more everything is compressed into one act and into one movement.
This latter appears to be derived, not original with On War, and Archer Jones’ analysis operates within that derivation.
Next: Theatres and Scott’s application of Clausewitz’s Sammlung.