function firstDayInPreviousMonth(yourDate) {
var d = new Date(yourDate);
d.setDate(1);
d.setMonth(d.getMonth() - 1);
return d;
}
EDIT: Alright… I’ve definitely learned something here. I think that this is the simplest solution that covers all cases (and yes, it does work for January):
function firstDayInPreviousMonth(yourDate) {
return new Date(yourDate.getFullYear(), yourDate.getMonth() - 1, 1);
}