(1) All taxes imposed under the provisions of this article shall be paid on the fifteenth day of the fourth month following the close of the taxable year; but the executive director may grant any taxpayer, upon application therefor, an extension of time for the payment of the tax, or any portion thereof, with interest to be charged on the unpaid balance at a rate imposed under section 39-21-110.5 for the period of such extension. No extension of time shall be authorized for payment of amounts of tax due upon deficiency assessments, or on amended or delinquent returns.(2) Payment of the estimated income tax or any installment thereof shall be considered payment on account of the income taxes imposed by this article.L. 64: R&RE, p. 795, § 1. C.R.S. 1963: § 138-1-73. L. 81: (1) amended, p. 1865, § 8, effective June 8.