I think it is very important to be familiar very history of music : it helps to understand, appreciate and interpret better music.
Of course, era determination is always kind of subjective : composers did not decide "OK, the old era is done, let's start a new one...", but it's rather a progressive evolution, a blending. And some composers do not belong to the main features of their era.
My advice to you : grab a good book about music history, do research, it's worth it.