Why does Douglass have no knowledge of his birth date?

From the text:

By far the larger part of the slaves know as little of their ages as horses know of theirs, and it is the wish of most masters within my knowledge to keep their slaves thus ignorant. I do not remember to have ever met a slave who could tell of his birthday. They seldom come nearer to it than planting‐time, harvesttime, cherry‐time, spring‐time, or fall‐time.