I just asked the Taipei tax office clerk about this, and according to her the calculation formula should only use the number of resident days when it’s less than 300. Above that, the multiplier should just be 365/365.
She also said that they’d correct that later if the software had applied the formula incorrectly, so consider yourself warned @qwert_zuiop 
I hope they’ll at least void the penalty if their own software is doing the calculations wrong… And I’ll also need to pay special attention that they will also adjust the deductions and not only the income (both were automatically adjusted by the software…)
Incidentally, the lady in the tax office also had no idea what those fields in the offline software were referring to when I showed her the screenshot in my above post:
It’s this kind of thing that makes me unsure of the correctness of info given by the tax office sometimes when it comes to less conventional situations like ours (despite them mostly being helpful and friendly).
I did try asking her about the thread topic (work done by Taiwan residents while physically outside Taiwan) while I was there. This was another question she wasn’t able to give a good answer to and the conversation devolved into some frustrating back-and-forth where we were both talking about different things, but the impression I got was that (i) it would be possible in principle to disregard income earned while physically outside Taiwan, but it would require additional evidence that the income was actually earned and/or paid while outside Taiwan (e.g., entry/exit records, documents from overseas tax authorities, possibly notarized stuff) and (ii) this would be easier in the 183–300 day range to meet the <300 days requirement mentioned above.
No definitive answer, though – we were both getting a bit irritated with the conversation at that point and it didn’t make enough difference to me to want to pursue it. (She was also looking at it from the perspective of someone with a full-time employer, and she didn’t seem to grasp how somebody might work remotely for, say, a couple of afternoons while on “vacation” or outside Taiwan for two months.)
Yeah, that’s why I am filing online - at least I am in control what I write down…
In my case, it’ll probably end similar to last year when I ended up going to the tax office after filing online to straighten out everything…
Just remembered - another thing she asked me about was why my foreign currency amounts converted into TWD at the tax office rates didn’t exactly match what the filing software had calculated. Mine were slightly higher than those calculated by the filing software, which meant I paid slightly less tax than I should have done… because the filing software ignores decimal places in foreign currencies and just takes the whole number.
I told her I’d give her the missing few TWD if she wants, but like… that’s not my fault, it’s your software. 
Everyone deserves a little break
It’s better in your pocket…
Did you ever figure out an answer to this? This thread got me thinking about the opposite situation, declaring income from a foreign employer that was earned before moving to Taiwan as Taiwan sourced income in order to meet the minimum income requirements for APRC.
For example, if someone had a gold card valid from Oct 2023 - Oct 2026, but only briefly visited Taiwan for short trips in 2023 and 2024 and didn’t move to Taiwan permanently until April 2025, they’d meet the minimum residency requirements of an average of 183 days per year over three years. (183 * 3 = 549 days, April 2025 - Oct 2026 is more than 549 days) Source: https://www.immigration.gov.tw/media/103510/0903-guidelines-for-foreign-nationals-applying-for-permanent-residency.pdf (section III.VII)
They wouldn’t be a tax resident for 2023 or 2024 because they spent under 90 days in Taiwan, but they would be a tax resident for 2025 and 2026 because they spent over 90 days.
If they worked in their home country from Jan 2025 - March 2025, before quitting and moving to Taiwan, could that money be counted as Taiwan sourced income by declaring it and paying taxes even if it was earned from a foreign employer while the person was living and working abroad? The justification, (not sure if it’s a good one), would be that spending 90+ days in Taiwan in 2025 makes you a tax resident for all of 2025 including the first half of the year before moving to Taiwan.
My reading of the APRC requirements, is that you can count income either for the preceding 12 months (ie sept 2025 - sept 2026) OR for the preceding calendar year (ie Jan 2025 - Dec 2025. If that’s the case, declaring the foreign income in 2025 could be helpful for meeting the minimum income requirement. Source: https://www.immigration.gov.tw/media/103510/0903-guidelines-for-foreign-nationals-applying-for-permanent-residency.pdf (section IV.(I).6.(1))
Curious if anyone has looked into anything similar and what the official guidance is.
Note that it’s 183+ days, not 90+ days to become a tax resident.
You do owe taxes in Taiwan when staying 90-183 days, but you’re not counted as a resident. But only for Taiwan-sourced income in this case - not for overseas income.
I highly doubt that. Taiwan-Sourced income is income for services rendered while physically in Taiwan - the tax office is even known for only partially counting overseas income as Taiwan-sourced if one stays less than 300 days per year in Taiwan.
From my understanding, the income earned while staying outside Taiwan should be filed as overseas income (with an exemption of about 6.7 million NT$ per year and a filing exemption of 1 million NT$) - and only applies if one is a tax resident, i.e. a foreigner staying > 183 days. Practically speaking, I doubt that many people bother to file income in this category (which would also include capital gains and overseas dividends) at all if they stay between 1-6.7 million or even above.
Note that the situation might also change when you lived in a country which has a tax treaty with Taiwan. Then, it gets more complicated. Personally, I avoided this situation by moving to Taiwan in Mid-October and then taking an extended vacation for the remainder of the year, so I would stay in Taiwan for less than 90 days that year and thus wouldn’t have to deal with any kinds of double-taxation agreements.
Of course, the tax office might not mind if you go ahead and say that it’s all Taiwan-sourced income. In the end, it means more tax revenue for them. But Taiwan being Taiwan, I wouldn’t be surprised if they actually argued against that…