Section 21. The assessors shall assess state and county taxes duly certified to them. The assessors shall also assess as estimated in advance by the commissioner charges and assessments for which the state treasurer will be required under section twenty to send his warrant not later than November first next following the assessment by the assessors. If at the time of fixing the tax rate for a fiscal year the warrant for the state tax, if any, for such year has not been received or the county tax for such year has not been certified, the assessors may nevertheless fix the tax rate for such year and shall in such event assess for such year such state and county taxes as estimated in advance by the commissioner. The commissioner shall notify the assessors of his estimate of such assessments, charges and taxes annually on or before March first. Any balance of such assessments, charges and taxes not so assessed for such year by reason of an underestimate by the commissioner shall be assessed for the next subsequent fiscal year and any excess assessed in accordance with the foregoing provision by reason of an over-estimate by him shall be deducted in determining the amount to be assessed for the next subsequent fiscal year. The assessors shall also assess town taxes voted by their respective towns and all taxes duly voted and certified by fire, water, light and improvement districts therein or certified by the joint committee of a health district formed under section twenty-seven A of chapter one hundred and eleven. Such district taxes shall be subject to the law relative to the assessment and collection of town taxes, so far as applicable. Except as otherwise provided, all taxes shall be assessed as of January first preceding the fiscal year with respect to which the taxes are assessed.