} else if (this._periodicity === 'semester') {
        var month2 = (this._value.substring(6, 7) - 1) * 6 + 1;
        if (month2 < 10) month2 = '0' + month2;

        this._firstDate = new Date(this._value.substring(0, 5) + month2 + '-01T00:00:00Z');