Calculate total per quarter of the year

There are ways to do this by formula but BY FAR the quickest and easiest way is to use a Pivot Table. It literally takes less than a minute. The Pivot Table will automatically take care of the aggregating by quarter (or year or month, etc).

1 - Click in the the table with the data

2 - Choose Organize > Create Pivot Table > On Current Sheet from the menu.

3 - In the Pivot Options tab drag the fields into the boxes below as shown:

4 - Click the circled i next to Date and choose Group By: Year-Quarter

Result:

If you need specific formatting for prettier presentation than you can get in a Pivot Table then you can easily copy-paste the visible cells in the Pivot Table into another existing "regular" Numbers table and format them there. Select, command-c, click once in a cell a destination table and command-v or Edit > Paste and Match Style to paste.

SG