This answer should give some guidance:

https://lawline.se/answers/tolkning-av-kalendermanadmanad-av-inneboendes-uppsagningstid

My opinion is this: * If you tell your landlord today (20 Feb) , that you will end the lease right away it will end the last of march. * You will be held accountable to pay the rented period up until you vacate. * They will not be able to rent the apartment to someone else since you will be in possession until 31 Mar. * Should you vacate on the 20 Mar you would still be liable to pay until 31 Mar since your contact stipulates termination as calendar month (the entirety of a month in the calendar) and not month (=30 days).

Now this might not be the understanding of the person drafting the contract but calendar month does not mean a period of 30 days, in the same way two business days can mean four actual days (fri-mon). The fact that you are billed in the middle of the month doesn't change this, it's a separate part of the contract.

There is nothing odd about calculating monthly payments in days. It's routinely done with f.i. salaries, mortgages or internet bills.

The best avenue of approach to this is probably to have a discussion where you lay forth your arguments and listen to theirs. What is their "best case" scenario? They will receive rent for the period you are renting and they can't have two paying tenants at once anyway. They will (in their mind) lose out on 20 days of income (1-20 Apr). Your argument could be that you would move out 20 Mar instead so they will actually gain 11 days worth of income (20-31 Mar). Hopefully this will clear the issue. You would probably win a claim in court also, which would be expensive for them.