Chapter III says that this happened on "a resplendent morning of early June".  Further, we read that "by the end of May" Tremayne and O'Moy thought he was dead.

But this is absurd.  The revelation to Lady O'Moy was "about a month" after O'Moy got the news, which he got "about a week" after the event [Chapter II].  Furthermore, O'Moy and Samoval duel at the end of May.