Before Present BP years is a time scale used mainly in archaeology , geology and other scientific disciplines to specify when events occurred in the past. Because the "present" time changes, standard practice is to use 1 January as the commencement date of the age scale, reflecting the origin of practical radiocarbon dating in the s. The abbreviation "BP" has alternatively been interpreted as "Before Physics";  that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable. In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating.
Radiocarbon Tree-Ring Calibration
How Do Archaeologists Count Backward Using BP?
In almost all archaeology books and articles the authors use dates. This is the Christian era in the Gregorian calendar, starting from 1 AD as the year in which Christ was believed to have been born. The date was calculated about years after the event, so was a broad estimate. If lower case letters are used, this often means that the date is based on an uncalibrated radiocarbon date see below for date calibrations. Battle of Hastings was in CE.
What Does cal BP Mean?
Radiocarbon dating can easily establish that humans have been on the earth for over twenty thousand years, at least twice as long as creationists are willing to allow. Therefore it should come as no surprise that creationists at the Institute for Creation Research ICR have been trying desperately to discredit this method for years. They have their work cut out for them, however, because radiocarbon C dating is one of the most reliable of all the radiometric dating methods.
Radiocarbon dating measurements produce ages in "radiocarbon years", which must be converted to calendar ages by a process called calibration. Radiocarbon years ago may be abbreviated " 14 C ya"  years ago or "uncal BP"  and calibrated dates as "cal BP". To produce a curve that can be used to relate calendar years to radiocarbon years, a sequence of securely dated samples is needed which can be tested to determine their radiocarbon age.