“O People of the Book! Our Messenger has now come to you, making things clear for you after a long break in the line of messengers, lest you say, ‘No bearer of glad tidings and no warner ever reached us.’ So a bearer of glad tidings and a warner has indeed come to you, and Allah has power over all things.” ⦗Al-Māʾidah 5:19⦘

This interval between the Messiah and Muhammad—may Allah’s blessings and peace be upon them both—was, as more than one scholar such as Salmān al-Fārisī and others have reported, six hundred years. Some say: six hundred solar years, which correspond to either six hundred and twenty or six hundred and eighteen lunar years, for every one hundred solar years amount to one hundred and three lunar years, as the Exalted says:

“And they stayed in their cave three hundred years, and added nine more.” ⦗Al-Kahf 18:25⦘

These nine—and a small part of a tenth—explain the difference. In historical reckoning full years are sometimes counted, while at other times partial years are included as well: whoever said “twenty” counted the incomplete years, whereas whoever said “eighteen” counted only the complete ones.