“I have a spreadsheet with dates and on each day, there are several activities with start time and end time. I want to add a row between each day, and also to add up the total time for all the activities in each day.”
This is where the Subtotals feature is quite useful.
It can add a variety of footers to appear at breaks in the group. You get outlining too.
Select your range, then from the Data menu, choose Subtotals…
At each change in StartDate, Use function Sum, Add subtotal to Duration.