At the bottom of your Balance Sheet Table is a section on Earnings (The famous "bottom line": sales less costs of sales and expenses) and Retained Earnings (Earnings (or losses) that have been reinvested into the company, not paid out as dividends to the owners). Business Plan Pro follows standard accounting convention in handling Retained Earnings. The amounts in current-period earnings ("Earnings'" in this Table) are moved into Retained Earnings once a year, at the end of the year.
Cell C30 (January Retained Earnings) contains a formula which adds Earnings from the Starting Balance to Retained Earnings in the Starting Balance. In our example, since the "Earnings" in the Starting Balance were $0, the Retained Earnings in January are the same as those in the Starting Balance.
This formula also subtracts any Dividends from the Cash Flow table (although in the example there are none). The formula in cell C30 (January Retained Earnings) is =B30+B31-dividends.
To abide by GAAP (Generally Accepted Accounting Principals) standards, Retained Earnings Changes Yearly, not Monthly.