▁radi oc ar bon ▁d ating ▁come ▁in ▁two ▁kinds : ▁un cal ibr ated ▁( also ▁called ▁Lib by ▁or ▁raw ) ▁and ▁cal ibr ated ▁( also ▁called ▁Cambridge ) ▁dates . ▁ ▁Un cal ibr ated ▁radi oc ar bon ▁dates ▁should ▁be ▁clearly ▁noted ▁as ▁such ▁by ▁" un cal ibr ated ▁years ▁B P ", ▁because ▁they ▁are ▁not ▁identical ▁to ▁calendar ▁dates . ▁This ▁has ▁to ▁do ▁with ▁the ▁fact ▁that ▁the ▁level ▁of ▁atmos pher ic ▁radi oc ar bon ▁( car bon - 1 4 ▁or ▁ 1 4 C ) ▁has ▁not ▁been ▁strictly ▁constant ▁during ▁the ▁span ▁of ▁time ▁that ▁can ▁be ▁radi oc ar bon - dated . ▁Un cal ibr ated ▁radi oc ar bon ▁ages ▁can ▁be ▁converted ▁to ▁calendar ▁dates ▁by ▁means ▁of ▁cal ibration ▁curves ▁based ▁on ▁comparison ▁of ▁raw ▁radi oc ar bon ▁dates ▁of ▁samples ▁independently ▁dated ▁by ▁other ▁methods , ▁such ▁as ▁d end ro chron ology ▁( dating ▁on ▁the ▁basis ▁of ▁tree ▁growth - r ings ) ▁and ▁strat ig raph y ▁( dating ▁on ▁the ▁basis ▁of ▁sed iment ▁layers ▁in ▁mud ▁or ▁sed iment ary ▁rock ). ▁Such ▁cal ibr ated ▁dates ▁are ▁expressed ▁as ▁cal ▁B P , ▁where ▁" cal " ▁indicates ▁" cal ibr ated ▁years ", ▁or ▁" calendar ▁years ", ▁before ▁ 1 9 5 0 . ▁ ▁Many ▁scholar ly / s cient ific ▁journ als ▁require ▁that ▁published ▁cal ibr ated ▁results ▁be ▁accompanied