Halliday & Matthiessen (2014: 406):
What is remarkable about serial tense is its regularity: the way in which each choice of tense, whether past, present or future, defines a location in time which is then used as the point of departure for a further choice among the same three tenses. This regularity is obscured, and distorted, by the categories of the structuralist analysis, and especially the ‘aspect’ nomenclature of perfect and progressive (or continuous).