Interest rate on tax bills

Currently in Zurich Kt interest on unpaid final bill is 4.5%.

Q1: From which date is the interest calculated? From due date of the final bill or from the end of the tax year that the bill refers to or some other date?

Q2: how is it calculated proportionally? E.g. is it 4.5%/365 per late day or some other formula?

The final bill has a 30 days grace period. It never came to my head to think that any interests might be accounted for before that period is over...

I think it's 4.5% annual, but the actual capitalization might be daily, monthly, etc, just saying (most likely daily as you indicated)

On the plus side: interest paid in the current year can be claimed as expense on the tax return for this year

(not that it is much of a benefit .. but still)

I didn't know this. So I got some interest in the final bill for 2021 that I paid this year as the final bill was received in 2023. So when doing tax declaration for 2023 I can deduct the interest for 2021?

Yes, that should be possible. As long as it was paid during the 2023 tax year.

Seems noone waited to see the effects of interest on not paid final bill

Not that I would encourage. I just need to sell off some stuff to pay the bill. Actually I did sell but still need the money to arrive from the brokerage acount and it's taking some time already...