Summing the algorithm datedif

HELLO
I HAVE A PROBLEM OF CALCULATION! I USED THE FUNCTION DATEDIF TO FIND THE DIFFERENCE BETWEEN MANY DIFFRENT DATES AND THE RESULT IS THESESONE " .... years...months..days " FOR EACH CALCULATE WHICH IS NORMAL. THE PROBLEM IS THAT I WANT TO USE THE FUNCTION IF SOIF THE VALID IS TRUE IT CAN BE SUMMED I WANT TO SUM ALL THESE ".....years....months....days" BUT I CANNOT FIND A WAY TO DO THAT

.....years..........months.........days" false

.....years..........months.........days" true

.....years..........months.........days" true

total years total months total days if true sum!!

THANK YOU