(a) In case of failure to file an income tax return on the date prescribed for the filing, such date to be determined with regard to any extension of time for filing, there shall be added to the amount of tax required to be shown on the return 5 percent of the amount of the tax if the failure is for not more than one month with an additional 5 percent for each additional month or fraction of a month during which the failure to file continues. No penalty shall be assessed pursuant to this Code section which exceeds in the aggregate 25 percent of the amount of the tax. No penalty shall be assessed pursuant to this Code section when it is shown that the failure is due to reasonable cause and not due to willful neglect.

(b) For the purposes of this Code section, the amount of tax required to be shown on the return shall be reduced by the amount of any part of the tax which is paid on or before the date prescribed for payment of the tax and by the amount of any credit against the tax which may be claimed on the return.

(c) With respect to any return, the amount of the addition under subsection (a) of this Code section shall be reduced by the amount of the addition under paragraph (1) of subsection (a) of Code Section 48-7-86 for any month to which an addition to tax applies under both subsection (a) of this Code section and paragraph (1) of subsection (a) of Code Section 48-7-86.

(d) No penalty due to late filing shall be incurred by a taxpayer if the taxpayer attaches to his return a copy of an approved extension of time within which to file his federal income tax return which has been granted by the Internal Revenue Service and also files his state return within the period of time specified in the extension. In such instances, the taxpayer need not apply to the commissioner for an extension of time within which to file his state return.