This date is approximate, but can't be far off.  We know that at least five days have passed since Rivarol's entry into the city on April 5.  ("In the course of four days …", Chapter XXVII, and the betrayal occurred the next morning.)  So the pursuit did not start before April 10.  Indeed, there's no reason to think it was any later.  But two days afterward Lord Willoughby says that William III has been King "for two months and more", so that day is no earlier than April 14.