The following is from Wikipedia which I think is relevant to your text. Wikipedia does make mistakes, and the phrase at the end contradicts their explanation, but I think it is still useful.

And their footnotes can take you to more original sources if you want to look at them.

http://en.wikipedia.org/wiki/Before_Present


Radiocarbon calibration
Further information: Radiocarbon dating#Calibration

"Dates determined using radiocarbon dating come in two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates.[12]

Uncalibrated radiocarbon dates may be expressed using BP years; however, they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon (carbon-14 or 14C) has not been strictly constant during the span of time that can be radiocarbon-dated.

Uncalibrated radiocarbon ages can be converted to calendar dates by means of calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating on basis of tree growth-rings) and stratigraphy (dating on the basis of sediment layers in mud or sedimentary rock).

Such calibrated dates are expressed as cal BP."

The following from Wikipedia contradicts what they say above:

", where "cal" indicates "calendar years" or "calibrated years"."