when adding 2 dated and one is blank it returns a value 42354

I have 2 columns each with a date in the 3rd column is my formula that subtracts one date from another and displays the remaining days but if one of the date columns is bland it returns a numeric value 42354 How do I add an if blank display nothing in my formula?

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=if(a1="", "", b1-a1)

One way...

=IF(COUNT(A1,B1)<2,"",B1-A1)