JS obtient le premier et le dernier jour du mois précédent
var d = new Date();
d.setDate(0); //sets d to the last day of the previous month
d.setDate(1); //sets d the the first day of that month
d.setHours(0,0,0,0); //sets d time to midnight
//d now equals the first day of the month before today
Lazy Lemur