Просто замените свою функцию
onDateSelection(date: NgbDate) {
if (!this.fromDate && !this.toDate) {
this.fromDate = date;
} else if (this.fromDate && !this.toDate && date.after(this.fromDate)) {
this.toDate = date;
} else {
this.toDate = null;
this.fromDate = date;
}
}
С
onDateSelection(date: NgbDate) {
if (!this.fromDate && !this.toDate) {
this.fromDate = date;
} else if (this.fromDate && !this.toDate) {
this.toDate = date;
} else {
this.toDate = null;
this.fromDate = date;
}
}