fortnight的含义为何和night无关？

我们知道fortnight为两周的意思，可它的结尾明明有个night，为何整个词的含义却和night(夜晚)毫无关联呢？
在词典中查阅该词的来源有如下表述：

17 c. contraction of M.E. fourteniht, from O.E. feowertyne niht, lit. "fourteen nights," preserving the ancient Gmc. custom of reckoning by nights, mentioned by Tacitus in "Germania" xi.

现在，我们知道fortnight为中世纪英语fourteniht的缩写，来自于古英语feowertyne niht，字面意思就是"fourteen nights"。

那问题来了，"fourteen nights"也是指14个晚上呀，那为何现在不表示和晚上(night)有关的意思呢？

preserving the ancient Gmc. custom of reckoning by nights, mentioned by Tacitus in "Germania" xi.

词典中继续阐述了它的由来，这种说法其实是保留了日耳曼语族用夜晚来计算天数的传统，所以这个词最终指的是天数：fortnight = fourteen night = 14天 = 两星期