Tenant prorated rent

[Hieu Ngo]

Location Minnesota

I had a year lease with my tenants that I did not renew. Payments were to made on the first of the month with a 5 day grace period. I chose let them stay on a month-to-month basis as a holdover tenant.

They provided notice on April 20 by phone that they were leaving by May 18. I told them to send me a formal letter which was postmarked May 24.

I just received a prorated rent check for 18 days based on 31 days instead of 30 days.

I believe I'm entitled to the full month's rent. I also fairly certain I'm going to have issues with the security deposit regarding changing paint on the walls.

My questions:
1. Am I correct in assuming that full month's rent is due even if they are leaving early?
2. Should I move to evict?
3. I think I'm going to Small Claims regardless, any advice?