Inside your PPOL account we use a so-called accounting calendar to partition reports along date lines and for some of the opportunity forecasting.
If you see a notice to ?Update your Accounting Calendar? it simply means you need to refresh the calendar period covered by the existing accounting calendar.
Click here to see a 1:30 minute video on setting up your Accounting Calendar
It is easy enough to get to. Simply go to Account (or MyAccount) > ERP Backend > Accounting Calendar Periods Setup