Historically, different dating systems have been used as well. This system works well for dates for archaeological sites, but it does pose problems for timelines. It can also be hard for non-archaeologists to understand. Year 0 under this system is actually 1950. So in 2000 A.D. you need to add 50 to any B.P. date.
Most archaeologists also use dates based on radiocarbon testing.
This scientific measure of age is based on changes in the radioactivity of a carbon isotope C-14. Properly done, it is a very accurate way of establishing the relative age of organic materials up to about 40,000 years old. Unfortunately for non-archaeologists, radiocarbon years are not exactly the same as calendar years. For example, a radiocarbon date of 10,000 BP actually corresponds with a date of about 11,250 to 11,450 years ago in calendar years, and 11,500 BP in radiocarbon dating represents about 13,150 to 13,550 calendar years ago. This is why some archaeological dates may differ from source to source depending upon what dating system is being used.