It is mandatory for all companies to have financial year as defined under Companies Act 2013 unless otherwise permitted by NCLT. Hence you cannot close the books on 31st March of same year. However, as regards IT Act, Sec.3 defines previous year as under:
3. For the purposes of this Act, "previous year" means the financial year immediately preceding the assessment year :
Provided that, in the case of a business or profession newly set up, or a source of income newly coming into existence, in the said financial year, the previous year shall be the period beginning with the date of setting up of the business or profession or, as the case may be, the date on which the source of income newly comes into existence and ending with the said financial year.
Hence previous year can be extended till the business is set up or source of income comes into existence.