Adjusting entries are made at the end of the accounting period (but prior to preparing the financial statements) in order for a company's accounting records and financial statements to be up-to-date on the accrual basis of accounting. For example, each day the company incurs wages expense but the payroll involving workers' wages for the last days of the month won't be entered in the accounting records until after the accounting period ends. Similarly, the company uses electricity each day but receives only one bill per month, perhaps on the 20th day of the month. The electricity expense for the last 10-15 days of the month must get into the accounting records if the financial statements are to show all of the expenses and the amounts owed for the current accounting period. Other adjusting entries involve amounts that the company paid prior to amounts becoming expenses. For examples, the company probably paid its insurance premiums for a six month period prior to the start of the six month period. The company may have deferred the expense by recording the amount in the asset account Prepaid Insurance. During the accounting period some of those premiums expired (were used up) and need to appear as expense in the current accounting period and the asset balance reduced.
Closing entries are dated as of the last day of the accounting period, but they are entered into the accounts after the financial statements are prepared. For the most part, closing entries involve the income statement accounts. The closing entries set the balances of all of the revenue accounts and the expense accounts to zero. This means that the revenue and expense accounts will start the new year with nothing in the accounts—allowing the company to easily report the new year revenues and expenses. The net amount of all of the balances from the revenue and expense accounts at the end of the year will end up in retained earnings (for corporations) or owner's equity (for sole proprietorships). Thanks to accounting software, the closing entries are quite effortless.