The taxpayer discount will not be allowed on returns which are filed after the twentieth of the month following the period covered or if t he return is not fully paid. The granting of extensions of time in which to file returns does not extend the period for claiming the discount. In the instance a taxpayer files and pays a timely return claiming a discount, and then subsequently files an am ended return owing more tax, the discount claimed shall be added back in the amended calculation.
35 Miss. Code. R. 4-01-02-105