Wis. Stat. § 74.15: Payment of real property taxes by grantor and grantee.

If real property is conveyed and there is no valid written agreement between the grantor and the grantee concerning the payment of real property taxes for the year in which the conveyance is made, the grantor shall pay to the grantee an amount equal to one-twelfth of the taxes assessed against the property for the calendar year preceding the year in which the conveyance is made multiplied by the number of months in the calendar year of the conveyance which have elapsed before the date of the conveyance, including the month in which the conveyance is made if the conveyance occurs after the 15th day of the month.