Martin (1992: 189-90):
There are a number of variations on as soon as, which shade into diversified realisations involving Circumstances and IDENTIFICATION. These include the moment that, the minute that, the second that and so on. This borderline area of realisation between the systems of IDENTIFICATION and CONJUNCTION will not be pursued here. Halliday & Hasan (1976: 230-1)and Halliday (1985: 308) suggest incorporating the "circumstantial" realisations into the account of CONJUNCTION to bring out proportionalities such as the following:
instead : instead of that ::as a result : as a result of that ::in consequence : inconsequence of thatetc.
But pursuing this is really a matter of perspective. From the point of view of IDENTIFICATION these expressions are phoric and combine with experiential clause systems to link clauses in a text; from the point of view of CONJUNCTION they are only minimally distinct from realisations involving conjunctions alone. It may be that Halliday and Hasan and Halliday are suggesting many of the realisations involving IDENTIFICATION are now congruent (that is, no longer processed as grammatical metaphors); this would be one reason for including them as part of the system of CONJUNCTION in the discourse semantics.
This is intended as an argument for determining which metafunction wordings such as the moment that, the minute that, the second that belong to — either to the textual discourse system of IDENTIFICATION or to the logical discourse system of CONJUNCTION. This is the same error as asking which metafunctional system a prepositional phrase belongs to, rather than asking what function it is performing with regard to each metafunction.