Section 2(11), which defines " previous year " in so far as it is relevant for purposes of this appeal is : " (11) (a) the twelve months ending on the 31st day of March next preceding the year for which the assessment is to be made, or, if the accounts of the assessee have been made up to a date within the said twelve months in respect of a year ending on any date other than the said 31st day of March, then at the option of the assessee the year ending on the day, to which his accounts have so been made up." 490 Section 3 of the Act provides: Where any Central Act enacts that income tax shall be charged for any year at any rate or rates, tax at that rate or those rates shall be charged for that year in accordance with, and subject to the provisions of, this Act in respect of the total income of the previous year of every individual, Hindu undivided family, company and local authority, and of every firm and other association of persons or the partners of the firm or the members of the association individually.