▁the ▁comm enc ement ▁date ▁( ep och ) ▁of ▁the ▁age ▁scale , ▁reflect ing ▁the ▁origin ▁of ▁practical ▁radi oc ar bon ▁d ating ▁in ▁the ▁ 1 9 5 0 s . ▁The ▁abbre viation ▁" B P " ▁has ▁been ▁interpreted ▁ret ros pect ively ▁as ▁" Before ▁Physics "; ▁that ▁refers ▁to ▁the ▁time ▁before ▁nuclear ▁weapons ▁testing ▁artificial ly ▁alter ed ▁the ▁proportion ▁of ▁the ▁carbon ▁is ot op es ▁in ▁the ▁atmosphere , ▁making ▁d ating ▁after ▁that ▁time ▁likely ▁to ▁be ▁un re li able . ▁ ▁In ▁a ▁convention ▁that ▁is ▁not ▁always ▁observed , ▁many ▁sources ▁restrict ▁the ▁use ▁of ▁B P ▁dates ▁to ▁those ▁produced ▁with ▁radi oc ar bon ▁d ating ; ▁the ▁alternative ▁notation ▁RC Y B P ▁is ▁explicitly ▁Radio ▁Car bon ▁Years ▁Before ▁Pres ent . ▁ ▁Us age ▁The ▁B P ▁scale ▁is ▁sometimes ▁used ▁for ▁dates ▁established ▁by ▁means ▁other ▁than ▁radi oc ar bon ▁d ating , ▁such ▁as ▁strat ig raph y . ▁ ▁This ▁usage ▁diff ers ▁with ▁the ▁recommendation ▁by ▁van ▁der ▁P licht ▁& ▁Hog g , ▁followed ▁by ▁the ▁Qu atern ary ▁Science ▁Re views , ▁both ▁of ▁which ▁requested ▁that ▁publications ▁should ▁use ▁the ▁unit ▁" a " ▁for ▁year ▁and ▁reserve ▁the ▁term ▁" B P " ▁for ▁radi oc ar bon ▁estim ations . ▁ ▁Some ▁arch ae olog ists ▁use ▁the ▁lower case ▁letters ▁b p , ▁b c ▁and ▁ad ▁as ▁termin ology ▁for ▁un