From MN Attorney General's link above (please confirm this is most recent law). I would think you need to calculate based on how long the tenancy has been in place based on chart below:
4. SECURITY DEPOSITS
Landlords have the right to require tenants to pay a security deposit (sometimes called a “damage deposit”). This is money paid by the tenant and held by the landlord to pay for any damage, beyond ordinary wear and tear, the tenant might do to the rental unit. It can be used to pay for any unpaid rent, or any money the tenant owes to the landlord under the lease or another agreement (e.g. water utility bills). (13) The security deposit cannot be used by the tenant to pay the rent. (14)
Amount of the Deposit
Minnesota law does not limit the amount a landlord may require as a security deposit. A landlord can increase the amount of the security deposit at any time during a “periodic tenancy” (a rental agreement in which no final date is mentioned), but only if the tenant is given proper advance written notice. Generally, this is one rental period plus a day.
If the deposit amount is stated in the rental agreement, and the rental agreement has a definite ending date, no changes in the deposit can be made unless both parties agree to the changes or the lease allows for changes.
At the end of the tenancy, the landlord must return the deposit to the tenant with interest. (15) Presently, the required interest rate is one percent (see the chart below). The landlord may keep the amount necessary to repair any damage done to the unit by the tenant (beyond ordinary wear and tear), or to pay off other debts related to the tenancy, including any unpaid rent. (l6)
Interest rate Time Frame
5 percent 8-1-73 to 9-30-84
5.5 percent 10-1-84 to 4-30-92
4 percent 5-1-92 to 3-21-96
3 percent 3-22-96 to 7-31-03
1 percent 8-1-03 to present