§613.  Fiscal year defined; accounting period not to be changed

"Fiscal year" as used in this Chapter, means an accounting period of twelve months ending on the last day of any month other than December.  However, no fiscal year will be recognized, unless, before its close, it was definitely established as an accounting period by taxpayer and the books of such taxpayer were kept in accordance therewith.  No accounting period shall be changed without the approval of the collector of revenue.