In Chapter 79 Dumas is rash enough to risk some actual dates (probably constrained by historical record :-p)
Aramis and Athos arrive at Boulogne on the first of February, after supposedly having been out of France for about six weeks in all (which as usual doesn't really match up with the amount of time recounted about their stay in England, unless the journey back to London with the captive King or the wait between the King's arrival in London and his execution occupied much longer than the events actually described by Dumas would imply -- probably the latter period lasted for more than the two or three days mentioned in the novel).
Aramis and Athos arrive at Boulogne on the first of February, after supposedly having been out of France for about six weeks in all (which as usual doesn't really match up with the amount of time recounted about their stay in England, unless the journey back to London with the captive King or the wait between the King's arrival in London and his execution occupied much longer than the events actually described by Dumas would imply -- probably the latter period lasted for more than the two or three days mentioned in the novel).