Adjusting entries are that account for transactions that occur in one period but affect another. You have to record these out-of-time entries as you get ready to close one accounting cycle to make sure they have been included in the right period.
Why we use adjusting entries
Sometimes transactions take place in one period but impact another. Other times, you know a transaction will be finalized in the future, but at least some part of it has ties to the current period. These two situations are the main reason for adjusting entries,
For example
To demonstrate the need for an accounting adjusting entry let's assume that a company borrowed money from its bank on December 1, 2010 and that the company's accounting period ends on December 31.
The bank loan specifies that the first interest payment on the loan will be due on March 1, 2011. This means that the company's accounting records as of December 31 do not contain any payment to the bank for the interest the company incurred from December 1 through December 31. (Of course the loan is costing the company interest expense every day, but the actual payment for the interest will not occur until March 1.) For the company's December income statement to accurately report the company's profitability, it must include all of the company's December expenses—not just the expenses that were paid. Similarly, for the company's balance sheet on December 31 to be accurate, it must report a liability for the interest owed as of the balance sheet date. An adjusting entry is needed so that December's interest expense is included on December's income statement and the interest due as of December 31 is included on the December 31 balance sheet. The adjusting entry will debit Interest Expense and credit Interest Payable for the amount of interest from December 1 to December 31.