=SI(C6="DCD";"TRM ";SI(C6="VIVANT";"VIVANT ";"OS "))&
SI(ENT(DATEDIF(B6;D6;"M")/12)=1;ENT(DATEDIF(B6;D6;"M")/12)&" an et ";SI(ENT(DATEDIF(B6;D6;"M")/12)>1;ENT(DATEDIF(B6;D6;"M")/12)&" ans et ";""))&
SI(MOD(DATEDIF(B6;D6;"M");12)>0;MOD(DATEDIF(B6;D6;"M");12)&" mois";"")