Why would a slaveholder want to keep a slave ignorant of such a simple thing as his birth date?

Slaveholders wanted to strip slaves of anything that might give the slaves a sense of identity.  Slaves were considered to be animals void of emotions, thought, or personality.  But slaveholders had to work at keeping the slaves at the level of animals.  They would restrict them from learning, reading, writing - they changed their names and kept them from knowing anything of their history or background.  By doing this, slaveholders hoped to keep slaves ignorant and downtrodden.  If a slave knew his date of birth, then he would know his age, and knowing how old you are is a part of having an identity.  If slaves developed an identity then they might gain the confidence to rise up against slaveholders.