June 1, 2018:
"Differential Historical Time" is your basic mouthful. Althusser produced this concept to think the way the different elements of a social formation evolve at their own tempos. Law, the state apparatus, ideology, culture, technology all have their own histories.
Think about conventional distinctions between historical periods. The Renaissance followed the Dark Ages. But that didn't happen over a single night, and it didn't happen in a way where each part of every social whole entered the Renaissance in lock step with the others. Think of the rebirth of Roman law beginning as early as the 12th century, then of the long lag of art and culture until their sudden rapid advance centuries later. Althusser's point is that elements of social wholes change at their own speeds, within their own relative autonomies.
Ben Brewster's glossary to Reading Capital detangles this:
"Time (temps). Hegelian theories of history see time as the mode of existence (Dasein) of the concept (Begriff). There is therefore a unique linear time in which the totality of historical possibilities unfolds. Empiricist theories of history as a chronology of ‘events' accept the same conception of time by default. This simple unilinear time can then be divided into ‘events' (short-term phenomena) and ‘structures' (long-term phenomena), or periodized in evolutionist fashion into self-contemporaneous ‘modes of production', the static or ‘synchronic' analysis of which has a dynamic or ‘diachronic' development in time into another mode of production. This dynamics or diachrony is then history. For Althusser and Balibar, on the contrary, there is no simple unilinear time in which the development of the social formation unfolds: each level of the social formation and each element in each level has a different temporality, and the totality is constituted by the articulation together of the dislocations between these temporalities. It is thus never possible to construct a self-contemporaneity of the structure, or essential section. Historical time is always complex and multi-linear. The synchrony of the social formation, or of one of its levels or elements, is the concept of its structure, i.e., of its dislocation and articulation into the totality. It therefore includes both ‘static' and ‘dynamic' elements (tendencies). The term diachrony can only be applied to the concept of the phase of transition. History itself is not a temporality, but an epistemological category designating the object of a certain science, historical materialism."
This is a similar idea to what Braudel and the Annales Group described as historical "durée", operating over long, medium, and short temporalities; and is pretty much what Trotsky termed "combined and uneven development". All these authors are seeking the same thing: a conception of historical temporality grasped in its fundamental unevenness. Think of Mandel's analysis of the Second World War which I quoted above. Durée, uneven development, combined and uneven development are all descriptions; differential historical time is the concept.