Conditional formula for date calculations

I am trying to construct a formula to calculate the number of days between 2 dates. If cell A1(starting date) is blank, return "0". If cell B1 (end date) is blank, return the difference between A1 and today's date. I have this: =ABS(TODAY()-E7)

Hi, an attempt. In E7-F7 the two dates

=if(isblank(E7),0,if(isblank(F7),today()-E7,F7-E7))

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or

=if(isblank(F7),today()-E7,(F7-E7)*(E7>0))

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Maybe it helps

Perhaps =(A1<>"")*(B1-A1)