Null Field and working with dates

In my Resets Table, I need the Equipment Ordered Date to be blank if there is not a numeric value in the Table/Field - Projects Order Equipment.
The field - Treat blank values as "0" is unchecked

And I have this formula to provide the date the equipment needs to be ordered by in my Resets Table
If(not IsNull([Project - Order Equipment]),WeekdayAdd([Start Date],-[Project - Order Equipment]))

The result I keep getting (and I have tried several variations) is the Start Date.​

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Hans Hamm
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