Halliday & Matthiessen (2004: 279-80):
But many items can occur both as circumstance and in one of the other functions [Modal or Conjunctive Adjunct]. In particular, prepositional phrases having a nominal group consisting of, or starting with, the word that are potentially either Conjunctive or circumstantial; thus, at that moment might well be a circumstance of Time in a history textbook ('at that moment in history') but conjunctive in a vivid personal narrative ('and just at that very moment'). What the grammar offers here, so to speak, are three planes of reality so that for (say) time, it construes experiential time, interpersonal time and textual time.
Experiential time is time as a feature of a process: its location, its duration or its repetition rate in some real or imaginary history.
Interpersonal time is time enacted between speaker and listener: temporality relative to the speaker–now, or usuality as a band of arguable space between positive and negative poles.
Textual time is time relative to the current state of the discourse: ‘then’ in the text’s construction of external reality, or in the internal ordering of the text itself.
Very often only the overall context will suggest which of the three is being foregrounded in a particular prepositional construction.