AlKhwārizmī is the first one to develop the concept of the algorithm in Mathematics. (jan 1, 800 – jan 1, 900)
Algorism is the technique of performing arithmetic with Hindu-Arabic numerals. In my opinion, AlKhwārizmī is worthy the title of "grandfather of Computer Science", since he was the one who takes the first step.
Added to timeline:
Zhang, Rui (Ella) - History of Computer Science