As not all opening balances are paid off in the first month, you can customise this.
You can do this by going to the balance sheet and clicking on the category i.e. trade debtors.
Then go to the opening balance page - you will see that the dropdown is currently set to default.
To customise your opening balance payment dates, change this option to invoice planned dates.
You will then see a table appears below where you can add details. You can add these details in groups i.e. amount to be collected in April/May etc or by individual invoices if you want more detail in your cashflow.
You can add the information manually or import it from excel.