Except as provided under § 58.1-3010, and except as provided by ordinance or special act in localities authorized to tax certain property on a proportional monthly or quarterly basis, tangible personal property, machinery and tools and merchants’ capital shall be returned for taxation as of January 1 of each year, which date shall be known as the effective date of assessment or the tax day. The status of all persons, firms, corporations and other taxpayers liable for taxation on any of such property shall be fixed as of the date aforesaid in each year and the value of all such property shall be taken as of such date, except that any county, city or town may permit a taxpayer to return as merchants’ capital the average amount of capital employed in his business on such date and on the next preceding August first.