Why is al-Ḥasan’s period included in the thirty-year count?

The Companions’ Disputes, Ijtihād, and the Rightly Guided Caliphate

• He held the caliphate briefly after ʿAlī
• He preceded Abū Bakr as caliph
• He established the ʿAbbāsid caliphate
• He was merely a later historian’s title