Hi. I am a B permit holder taxed at source. Every month my tax was mistakenly calculated according to the canton where I work, while I live in a different canton. I am aware that at the beginning of the new year I should ask for the tax recalculation. My question is: I have to do this before when? Someone told me the deadline is 31 March. Is that correct? Thanks.

The deadline varies from canton to canton so it really depends where you live.

Your HR dept has royally messed up. What do they say?

Unless you have to do a tax return (see other posts, generally if you more than 120k) you should be asking your HR dept to calculate the correct amount and credit you with the difference on your next payslip. It's their legal obligation to deduct the right amount and pass it to the authorities.