text: export function subMonth(month, checkDate) {
  	month = Math.abs(month); //plus sign before a variable, convert variable to int
  	const pastMonth = this.d.month; //pastMonth -> Number of month that is past
  	if (pastMonth > month) {
  		this.d.month -= month;
  	} else {
  		this.subYear(1, false);
  		this.d.month = 12;
  		month -= pastMonth;
  		while (month >= 12) {
  			this.subYear(1, false);
  			month -= 12;
  		}
  		if (month != 0) {
  			this.d.month -= month;
  		}
  	}
  
  	while (checkDate && !this.isValidDate()) this.subDay(1, false);
  	return this;
  }
  