What is meant by the word "Ad-Dahr" in the Hadith: "Whoever fasts Ramadan and follows it with six days of Shawwal, it is as if he has fasted Ad-Dahr"; does it mean a lifetime or a year, and what is the ruling for one who unintentionally breaks his fast during the six days of Shawwal; must he make up the fast or not?

The word " الدهر (time/period)" in the Hadith (definition) means a year, because a good deed is multiplied tenfold. Thus, fasting Ramadan (definition) is like fasting ten months, and fasting the six days is like fasting two months. So, fasting six days after the month (of Ramadan) becomes like fasting a full year. This is indicated by the Hadith: "Whoever fasts six days after 'Eid al-Fitr, it is as if he has fasted the entire year, for whoever comes with a good deed will have ten times its like."