How do I create a sum for each group of data in Libre Office Calc?

I was on the wrong path. What I wanted was Data->Subtotals. Then you pick the column to use as a control-break (in my case, Invoice_Date), the column that you want to subtotal on (in my case, Invoice_Amt), and what you want to calculate. (I wanted SUM(), but you could do AVG, MIN, MAX, etc)

One way would be to use a Pivot Table. Select the data table including the headings then choose Data/Pivot Table/Create/Current Selection. In the pivot table dialog click and drag the Invoice_date to the Row Fields section and the Invoice_amt to the Data Fields section. The default is to Sum the Invoice_amt. By default the pivot table will be put on a new sheet, but you can change that (and some other options) using the More button.