BC, the abbreviation is B.C., which means "Before Christ/Before the Lord", that is, before the first year of the Christian era. For example: 197 BC, 8 BC, etc. xx years BC, also referred to as "xx years BC".
AD is a chronology. The so-called 2014 AD is the 2014th year from the first year of AD. Then 2017 BC means the 2000th year before the first year of AD. Seventeen years, four thousand and thirty-six years from now (2019 AD). ?
The BC chronology is counted backwards. For example, the Western Han Dynasty was founded in 202 BC, and the Qin Dynasty was founded in 221 BC. 202 BC was later than 221 BC. But note that the months of each year in BC are still numbered, with January first and December last. The same goes for dates, with the first day of the lunar month coming first and the fifteenth day of the lunar month coming last.
Extended information:
The year 1 AD is equivalent to the first year of the Yuan Dynasty of Emperor Ping (Liu Chan) of the Western Han Dynasty in my country.
The word "century" comes from Latin, which means 100 years or hundreds. It is also calculated from the year of the birth of Jesus. The first century is from 1 to 100 AD, and the first century is from 101 to 101. 200 years is the 2nd century. By analogy, 2017 is the 21st century, and 2001 is the first year of the 21st century.
Because of the promotion of this chronology, after entering China, it gradually became different from the lunar calendar used by the people, and was called the "New Calendar" and the "Gregorian Calendar". Before the first year of the New Calendar, The era is naturally called "B.C." So "B.C." is another Chinese name for "B.C."