Let's say you have an event from the 5th to the 10th and you want these dates to show on your quote/ invoice.

First, click 'edit' on Project Info

Next, add in the Start and End time and click Save

Now when you go to create Line Item Groups, they will match the start and end dates.
FYI - Without the Start/End times, the Line Item Groups will default to today's date.

In addition to helping you time your Line Item Groups, the Start/End times will also appear on the quote and invoice, on the prompt when sending the quote, and on the custom column.