When did the reckoning of the hundred years mentioned in the Hadith, "Indeed, Allah sends to this Ummah at the head of every hundred years one who rectifies its religious affairs," begin?

The Hadith indicates the continuation of this worldly life and the survival of the Ummah (Muslim nation) after the Prophet, peace and blessings be upon him, as is evidenced by reality and affirmed by the Hadith: "A group among my Ummah will continue to be manifest upon the truth. Those who forsake them will not harm them, until the Command of Allah comes while they are in that state." However, this continuation is not eternal, for this worldly life is transient and will be followed by the eternal afterlife. As for the continuation of the Ummah, it will last until close to the Hour, when the souls of the believers will be taken by a gentle breeze, as stated in a Hadith in Muslim: "Indeed, Allah will send a breeze from Yemen, softer than silk, and it will not leave anyone in whose heart there is... any faith but that it will take their soul." Al-Nawawi clarifies that this means their perseverance upon the truth until this gentle breeze takes their souls near the Hour.