The date here is no more than a guess.  All we know for certain is that it must have happened after William's victory at La Hogue in May (where Lochmore was present) and before Glenleven hits on Ailsa at Christmas.

However, Glenleven waited until after "the period of mourning retirement … was at an end".  It seems unlikely that Ailsa's formal mourning lasted less than a couple of months—indeed, a year would be more seemly.  And Glenleven's attentiveness must then have extended at least a further month or two.  So I've placed the duel relatively early in the permissible range, hoping to be off by no more than a month one way or the other.