If the due date is always 100 days from invoice date (which would be unusual in practice, different customers would normally different payment terms) then teh sikmplest way is to use an SQL Computed column:
Specify Computed Columns in a Table | Microsoft Docs[
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A specification of
(DATEADD(day,(100),[InsertDate]))
would do it.
But I'd do it in C# using DateTime.AddDays instead as it allows for flexibility later.
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