Null Date Field

Hey Everyone,

I wanted to see if I could get some Formula advice. I have a formula that takes a start date and adds duration:

WeekdayAdd([Sub Start Date], ToNumber([Gas Pipe Total Duration])-1)

I had to include the -1 because I noticed it starts the count after the start date, but doesn't include the start date. 
But if the total duration is blank or 0 how can I have it not display a date? Right now it's showing the day after start date because of the -1, or just the start date if I remove the -1.

Any tips on this formula?

Thanks,

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Alex Bennett
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