Go to Administration>Configuration>Accrual Accounting Period Definition.
In the left section of the window, add a new row.
Select the Calendar Month radio button.
Enter the start date of the accounting year in the top of the right section. The year start date must be the first date of the month. If the date is not correct, the system will not allow you to define the Calendar Month method until you enter the proper date.
All the fields in the Periods grid are automatically completed starting from the date chosen as a year start date and you cannot modify them. For each calendar year, twelve periods are created by default and each period has duration of one month.
