at a time, at one time

take time 意为"花费时间”，

take one’s time 意为"慢慢来，别着急”，相当于 “don’t hurry”。

例如:

A great talent takes time to mature. 大器晚成。

Take your time, we still have 20 minutes left. 别着急，我们还有20分钟的时间呢。

at a time 意为"每次；一次”。

at one time 意为"同时；曾经一度”，常用于过去时。

at the time 意为"在那时；在那段时间”。常用于过去时态。

例如:

Take two pills at a time. 一次服两粒。

At one time they were friends, but they are seldom together now. 他们曾经是朋友，但是现在很少在一起了。

He agreed at the time but later changed his mind. 他当时同意了，但后来又改变了主意。