Tax and being over the barrel

Ok, I have an ARC based on marriage. Do I have to endure the 183 day 20% tax thing? IF I do, does this mean I will get it back next return?

Also, why does Taiwan base its monthly salary on 30 DAYS as opposed to 4 WEEKS? I got paid effectively half what I was expecting due to this… any insights??

I’m interested to learn more about this as well.