I would disagree. On of my first jobs was porting assembler to C for a paging switch. The assembler was well written and well documented. The task was slow, hard and painfully boring. It took months to get every function to be a perfect match.
In some ways it reminds me of the project to programatically translate the golang compiler from C to Go. Though I imagine the codebase is quite a bit messier!
But, my understanding is that the bulk of the work done in this case was done by a conversion tool that was designed and developed by a group of 8 people led by the gentleman referred to in the article (Jian Wang). The conversion tool project was the work that I was referring to in my comment.
I would disagree here -- like timezones, tax software has the disadvantage of being both extremely boring, and very fiddly (thus requiring close attention). This combination makes it very hard to maintain the concentration that's required to write exhaustive tests.