Halliday & Matthiessen (2014: 158):
What is common to the modal and conjunctive Adjuncts, as distinct from the circumstantials, is that they are both constructing a context for the clause. Thus even though the same semantic feature may be involved, for example time, it has a different significance in each case. A modal Adjunct of time, such as just, yet, already, relates closely to the primary tense, which is the ‘shared time’ of speaker and listener; a conjunctive Adjunct of time, such as next, meanwhile, locates the clause in time with respect to the preceding textual environment; and both are different from time as circumstance, such as in the afternoon. And the same item may function sometimes circumstantially and sometimes conjunctively; for example, then, at that moment, later on, again.