(a) The term interbank placements means the average value of interest-bearing funds, with a maturity of less than one year, placed or deposited by a taxpayer with a banking corporation other than one described in section 16-2.5(j) of this Title (whether or not a taxpayer), provided such banking corporation is not one:

(b) In determining average value of interest-bearing funds described in subdivision (a) of this section, the book value of such funds must be used. The average value of interest-bearing funds is computed on a daily basis. Where the taxpayer's usual accounting practice does not permit the computation of the average value of interest-bearing funds on a daily basis, a computation on a weekly basis will be permitted. The Tax Commission will not permit the computation of average value of interest-bearing funds on a basis less frequent than weekly, unless the taxpayer demonstrates that requiring it to use a weekly basis would produce an undue hardship. Any method of determining average value of interest-bearing funds which is adopted by the taxpayer on any return and accepted by the Tax Commission may not be changed on a subsequent return without the prior written consent of the Tax Commission.