The original passage (which I changed) said "a month goes by" (clearly wrong).  The script says Act I scene 1 (or 2, I forget which) takes place in May, and Mrs. Mullin's line is in June, as indicated before that point in the script.  May and June are only one month apart, so clearly 13 months have passed.  I suppose it could be 25 months, too, but one is not possible.