S.C. Code § 12-37-2620
The tax payable on motor vehicles required to be licensed by Section 56-3-110 must be determined for a twelve-month licensing period, except when the license required is for a twenty-four month licensing period. In that case the tax payable on motor vehicles required to be licensed by the department must be determined for a twelve-month tax year for each of the two twelve-month periods contained in the biennial licensure as they respectively occur. If the actual licensing period is less than twelve months for either of the two twelve-month tax years, the tax payable must be that proportion of the above described tax that is equal to the proportion of the number of months that the licensing period is to the twelve-month period that is affected.
S.C. Code § 12-37-2620