815 ILCS 205/10

In all computations of time, and of interest and discounts, a month shall be considered to mean a calendar month, and a year shall consist of twelve calendar months; and in computations of interest or discount for any number of days less than a month, a day shall be considered a thirtieth part of a month, and interest or discount shall be computed for such fractional parts of a month upon the ratio which such number of days shall bear to thirty.