Since it is a word that describes a time frame, rather than a particular style or unifying idea, there are no universally agreed on criteria for making these distinctions.

• 虽然说一个时间段很容易,但是要描述一种特定的风格或者统一的主张则不是一件容易的事,我们很难给这些区别给出一个普遍认同的标准。