Peter,

Thank you very much. So I created a new query and pasted your SQL string into the SQL view. When I first ran it, I got an error
Your query does not include the specified expression "Base_Due_Date" as part of an aggregate function.

Having no idea what that meant or what exactly to do to fix it, I started fiddling around. First I removed "tblProjects.Base_Due_Date" from the first line. When I ran the query I got no error and the query returned the ProjID and the sum of the extensions!!! I then went to design view and added "Base_Due_Date" and this time when I ran the query again I got exactly what I was looking for:

ProjID Base_Due_Date Total_Days_Extended
1 1/1/2024 40
2 2/1/2024 15
3 3/1/2024 20

When I went back and looked at the resulting SQL I noticed tblProjects.Base_Due_Date was added to the GROUP BY line.

SELECT tblProjects.pkProjID, tblProjects.Base_Due_Date, Sum(Nz(tblExtensions.NumDays)) AS Total_Days_Extended
FROM tblProjects LEFT JOIN tblExtensions ON tblProjects.pkProjID = tblExtensions.fkProjID
GROUP BY tblProjects.pkProjID, tblProjects.Base_Due_Date;

Why is the GROUP BY line necessary? What does Nz mean and what does it do?

Again thank you. Hopefully I will be able to apply what you have shown me here to a practical project that is much more complicated.