day length: how to calculate it and its effects on plant growth

In the UK day length affects development of several crops that are used to growing at much more equatorial lattitudes but survive here due to our relatively mild climate. Even potatoes have been gradually bred to from tubers at longer daylengths. Other crops this affects are Oca and Mashua, which both need a mild Autumn to be able to tuberise well before the frosts kill the plants.