Fix PLN amount rounding for compliance with PL tax rules 💰

[mbronk]

Polish tax law requires different rounding methods depending on income type, which was not previously implemented. This change (hopefully¹) brings the calculation into compliance with Art. 63 Ordynacji Podatkowej (OP).

1. Each currency-converted tax event (i.e. stock sales, dividend...) is expressed with "grosz" (0,01) precision - at the time of FX calculation, before aggregating further.
   Basis:
   • Art. 11a Ustawy o PIT only defines the FX conversion formula (NBP@T-1), but the Act does NOT prescribe special rounding rules,
   • 0,01zł is the lowest monetary value and sub-grosz values cannot be reported on any tax forms,
   • Therefore: Mathematical rounding rules apply ( >= 0.005zł --> 0.01zł).²

2. Interests and dividends are separated as they follow different rounding rules when calculating the lump-sum tax
   1. Interests aggregate (art. 30a ust. 1 pkt 3 PIT) as well as their resulting lump-sum tax, are rounded ⬆️UP to the full "grosz" 🪙 (0,00001zł -> 0,01zł) — art. 63 §1a OP
   2. Dividends aggregate (art. 30a ust. 1 pkt 4 PIT) as well as their resulting lump-sum tax, are rounded to the nearest full ZŁOTY 💵 (0,50zł -> 1zł) — art. 63 §1 OP
   3. Foreign tax withholding: no rounding; the standard FX rule to round to grosz (0,01) precision - rule 1 (above)
   4. Net/gross/cost stock proceeds are not subject to lump-sum tax calculations and reported in full on PIT-38 form, hence only standard FX rules (no. 1-above) applies and they are reported with "grosz" precision.

Warning

• Requires UT: Fix ECB TITLE to match current API value #179 to be merged first🥇, for CI to pass

Footnotes

1. Disclaimer: the stated interpretation of the rules reflects my best understanding of the current Polish tax acts and - in some cases - some non-binding advice I've received at Krajowa Informacja Skarbowa (KAS). I'm neither a tax professional, nor an accountant/financial advisor, so do NOT take these as source of "truth" as these clearly are just "works for my personal tax reports", not a tax or legal advice. In fact, I highly suggest reviewing the relevant acts (and/or reaching out to your Tax Office, etc.) and drawing own conclusions. ↩

2. § 5 ust. 6 rozporządzenia MF z 28 listopada 2008 r for VAT states:
   "Kwoty podatku wykazuje się w złotych bez względu na to, w jakiej walucie określona jest kwota należności w fakturze. Kwoty wykazywane w fakturze zaokrągla się do pełnych groszy, przy czym końcówki poniżej 0,5 grosza pomija się, a końcówki 0,5 grosza i wyższe zaokrągla się do 1 grosza." ↩

[mbronk]

• Potentially fixes Make sure each transaction is rounded to single cent/zloty/eurocent #109

[jczaja]

I had talked to tax advisor who then called "Naczelna izba skarbowa" to disscuss the matter of rounding. He told me that we should not round every single transaction just a sum of transactions.

[mbronk]

I had talked to tax advisor who then called "Naczelna izba skarbowa" to disscuss the matter of rounding. He told me that we should not round every single transaction just a sum of transactions.

Thanks for looking into this! I was aware of it being a little sketchy, but I haven't found any written rule preventing the rounding, and my objective was for the values to match the pen and paper calculations I was doing on a side, to confirm.¹

I've now changed the default so there's no per-transaction rounding now, consistent with what your advisor confirmed.

Since, to my knowledge, that guidance isn't formally documented anywhere, I kept per-transaction rounding as an opt-in² (--round-per-transaction / GUI Options toggle) for anyone who wants to match pen-and-paper calculations (it helps me be consistent with my own past-years calculations). It's off by default.

Footnotes

1. On a separate topic, I think use of f32 for financial data is a little risky, given the possible accumulated rounding error. I've switched to 'decimal' on my fork - will add it as a PR for your consideration. ↩

2. Of course, if there are reliable/quotable references proving my method is invalid, the extra option makes no sense and should be deleted. If you find anything - please do share! As long as it is "sketchy", I hope you'd be OK with this remaining an option for the user to select (aka. I prefer to continue to run my own reports w/ this enabled) ↩

[jczaja]

@mbronk Hi, thanks for your effort but I'm not going to merge this PR. The reason is that it introduces significant changes that will have to be maintained and tested. Moreover option in GUI will confuse many users who will keep coming for explanation as not all of them are familiar with details of tax calculation that are disscussed in this PR. So after all I do not think there is a valid cause to have this PR merged. Thanks for your effort.