Subtract two columns using conditions

I have two date columns, I want to subtract the two columns based on conditions. First check for all the blanks in the first column and then check second column for blanks and the third condition check if the subtracted dates are less than one. If these conditions are satisfied, carry out subtraction of the the two columns. Something like this:

'''if [Recommendation signed] = null or [Executed Date] = null or Duration.Days([Contract Executed Date]-[Recommendation signed]) < 1 then null else Duration.Days([Contract Executed Date]-[Recommendation signed])'''