If isblank

I have three columns a, b and c. I'm applying the formula =IF(ISNUMBER(B5+A5),DAY(B5) & "/" & MONTH(B5) & "/" & YEAR(B5),"") to take out the hrs, mins, sec component of the date and place it in column C.
I only want to publish the date in the Column C that has numerical values in Column A but 0/1/1900 is published in Column C when null values are encountered in column a.

I was looking into using OR IF(OR(ISBLANK(A)) but I can't get this to work with my formula.

Would anyone be able to help. Excel Book attached.