=SI($B2>"31/12/2020"*1;"";DATEDIF(MAX($B2;"01/01/2020"*1);SI($C2="";"31/12/2020"*1;MIN($C2;"31/12/2020"*1));"d"))
=SIERREUR(SI($B2>"31/12/2021"*1;"";DATEDIF(MAX($B2;"01/01/2021"*1);SI($C2="";MIN(AUJOURDHUI();"31/12/2021"*1);MIN($C2;AUJOURDHUI()));"m"));"")
=SOMMEPROD(--(ANNEE(LIGNE(INDIRECT($B2& ":" & MIN($C2;SI($C2="";AUJOURDHUI();$C2)))))=2020))
=MAX(0;SI(C2="";"31/12/2020";MIN(C2;DATEVAL("31/12/2020")))-MAX(B2;DATEVAL("1/1/2020"))+1)
=MAX(0;JOURS360(MAX(DATEVAL("31/12/2019");DATE(ANNEE(B2);MOIS(B2);0));SI(C2="";"31/12/2020";MIN(DATEVAL("31/12/2020");DATE(ANNEE(C2);MOIS(C2)+1;0))))/30)