Generally, you can use the cash method, accrual method, or any other method permitted by the Internal Revenue Code. In all cases, the method used must clearly reflect income. Unless you are a qualifying taxpayer or a qualifying small business taxpayer, you must use the accural method for sales and purchases of inventory items. Special rules apply to long-term contacts.
If you use the cash method, show all items of taxable income actually or constructively received during the year (in cash, property, or services). Income is constructively received when it is credited to your account or set aside for you to use. Also, show amounts actually paid during the year for deductible expenses. However, if the payment of an expenditure creates an asset having a useful life that extends substantially beyond the close of the year, it may not be deductible or may be deductible only in part for the year of the payment. See Chapter 1 of Pub.535.
If you use the accrual method, report income when you earn it and deduct expenses when you incur them even if you do not pay them during the tax year. Accrual-basis taxpayers are put on a cash basis for deducting business expenses owed to a related cash-basis taxpayer. Other rules determine the timing of deductions based on economic performance. See Pub. 538.
To change your accounting method, you generally must file Form 3115. You also may have to make an adjustment to prevent amounts of income or expense from being duplicated or omitted. This is called a section 481(a) adjustment.
Example: You change to the cash method of accounting and choose to account for the inventoriable items in the same manner as materials and supplies that are not incidental. You accrued sales the year before for which you received payment in the tax year. You must report those sales in both years as a result of changing your accounting methods and must make a section 481(a) adjustment to prevent duplication of income.
A net negative section 481(a) adjustment is taken into account entirely in the year of the change. A net positive section 481(a) adjustment is generally taken into account over a period of 4 years. Include any net positive section 481(a) adjustments on line 6 of the form. If the net section 481(a) adjustment is negative, report it in Part V.
For full details, see Schedule C Instructions.