Is Al-Hasan ibn Ali the fifth of the Rightly Guided Caliphs, and is there a prophetic text specifying the Rightly Guided Caliphate to be thirty years, ending with Al-Hasan's caliphate?

There is no doubt that Al-Hasan (definition) was a legitimate caliph who completed the period of the Caliphate. He assumed the Caliphate after Ali ibn Abi Talib, may Allah be pleased with him, on the seventeenth of Ramadan (definition) in the year 40 A.H., and he abdicated it to Mu'awiyah, may Allah be pleased with him, on the twenty-fifth of Rabi' al-Awwal in the year 41 A.H. Thus, his caliphate lasted for approximately six months, which was the period that completed thirty years after the death of the Prophet, peace and blessings be upon him. The reason for 'Umar ibn 'Abd al-'Aziz being titled the fifth Rightly Guided Caliph might be due to the deviation of the caliphs before him from the prophetic guidance, or due to the short duration of Al-Hasan's caliphate and the turbulent state of affairs among Muslims during that period.