When zero is reached, the timer starts counting up to show time elapsed since alarming.

在达到零秒时，计时器便开式正数计时来显示从报警后开始的时间。

youdao

One time he started counting backwards. he said something like '38', and i didn't get it, then he said '36'…wait a minute, wait a minute, he goin' backwards and when he gets to zero, he got 40!

有一次他（乔丹）开场得分后数道“38”，我没懂什么意思。 然后他数道“36”……等等，他是在倒数。 当他数到零时，他已经得了40分了。

youdao

It's how you can initialize a variable to like zero for counting's sake or something like that.

你可以初始化一个变量，像0用来,计数的或者诸如此类的。

哈佛公开课 - 计算机科学课程节选