public void incDay(int days) {
        if (this.day + days <= 30) {
            this.day = this.day + days;
        }
        else if ((this.day + days > 30 && (this.day + days)%30 == 0)) {
            this.month += (this.day + days)/30;
        }
        else if (this.day + days > 30 && (this.day + days)%30 != 0) {
            int count = (this.day + days)/30;
            this.day = (this.day + days) - count*30;
            this.month = this.month + count;
        }

        if (this.month > 12 && this.month%12 == 0) {
            this.month = 12;
            this.year += this.month/12;
        }
        else if (this.month > 12 && this.month%12 != 0) {
            int count = this.month/12;
            this.month = this.month - count*12;
            this.year = this.year + count;
        }
    }