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.