Answer
With a company selected in the online app, use the Company Notes or Calendar icon beside the company name at the top of the page.
Company Notes
Use Company Notes to keep dated notes for the selected company. Add a row, enter the Date and Text, then save it. You need permission to edit the company to add, change or delete a company note.
For notes about one employee, use Employees >> Details >> Notes instead.
Calendar
The Calendar gives you a quick date reference with week numbers and weeks starting on Monday. It does not show company events or pay dates.