The birth date refers to the year, month, day and hour of a person's birth, using the era of the heavenly stems and earthly branches (also called the Jiazi era). The year, month, day, and hour are represented by eight characters (also called four pillars) using heavenly stems and earthly branches.
Nowadays, births are based on the AD era. Just convert it into the celestial stems and earthly branches. For example, I was born at 16:28 on April 21, 2010. If I check the perpetual calendar, my birth date is Gengyin (year), Gengchen (month), Xinchou (day), and Yiwei (hour).