35 Miss. Code R. § 3-01-06-103
The initial return may be filed in accordance with the basis and method of accounting regularly used by the taxpayer, and that clearly reflects the taxpayer's income. Arbitrary changes in the basis or method of accounting employed by the taxpayer are not permitted except in cases where such changes have been approved in advance by the Commissioner. Where no accounting basis has been established by the taxpayer, or where the method employed does not clearly reflect income, and the taxpayer for whom the method is prescribed shall be bound by the same until such time as it is shown to the satisfaction of the Commissioner that a method proposed by the taxpayer will reflect such income with equally satisfactory results. In the absence of an established accounting period, the income of the taxpayer shall be computed on the basis of a calendar year.
35 Miss. Code. R. § 3-01-06-103