The balance sheet is the financial statement showing the health of a business from day one to the date on the balance sheet. So, unlike the income statement, which is for a specific time period (month, quarter, year) the balance sheet is all inclusive time-wise. That's why the header is worded As of 12/31/20XX with no date qualifiers.
Accounting is based upon a double entry system - for every debit there must be a credit. The net effect is zero which results in the balancing of the books. The proof of this balancing act is shown in the balance sheet when Assets = Liabilities + Equity.
