Sorry for wall of text, I really tried to analyze and solve the problem in depth.
Analyzing the problem
Don’t let yourself be blocked by uncertainty. You can put the uncertainty in a function and implement the more likely (or easier) case first. Ask questions in parallel. If it turns out that the other possibility was correct, you just refactor the implementation of this one function. The rest stays the same, so you can nearly finish all the work without knowing everything.
The question itself seems hard to answer, since normally ex-dividend and payment are tied together.
From a theoretical market view, the stock price loses the dividend amount on ex-dividend. Now this future dividend is owned by the ETF (and owed by the company). So, the ETF price shouldn’t change, even though the underlying price dropped by the dividend. If the ETFs creation and redemption process accounts for accrued dividends, I don’t know.
From the perspective of the tax administration, it seems the payment date is tax relevant. I couldn’t find any regulation, but the data you provided included the “Allstate Corporation”. Their dividend had the interesting property that it went ex-dividend on 2022-11-29 but was paid on 2023-01-03. ICTax now says this was taxable in 2023. So that is a good indication that the payable date counts.
This means that ex-dividend matters for the ETF and payable date matters for tax (but only for what was accrued on ex-dividend, because only that will actually be paid).
So both are important.
Let’s work backwards:
In the end you want to know per share of ETF for each day per country:
- gross dividends
- tax withheld (see complications #2)
For “gross dividends” you need to know at payable date:
- total ETF shares
- total gross dividends (per country)
For “total gross dividends” you need to know for each day per ISIN:
- total shares going ex-dividend
- gross dividend per share
- payable date of the dividend
- share domiciles
Edit: After getting gross dividends and tax withheld, you could additionally product-sum with the shares of ETF held each day to get a yearly number (per country).
Complications
#1
Tax can be withheld and then released afterward. For example, many ETFs announce the split of their own distributions between ordinary income, short-term capital gains, long-term capital gains only after the year has finished. Since capital gains are not subject to withholding taxes, your broker should get those back for you (and IBKR apparently does).
But if this happens inside an ETF, it could be that tax was withheld, but you sold the ETF before they got it back.
But capital gains are not taxable for natural persons in Switzerland. You just got tax credit on capital gains, but actually paid normal income taxes on it (and also raised your tax bracket). So you paid more taxes than you should have and that shouldn’t irk the tax administration too much.
Conversely, if you buy an ETF and then it gets previously withheld taxes back, you don’t pay taxes on that, because it is capital gains.
But all of this doesn’t hold if you are a legal entity.
#2
Looking at the data and your link, you don’t know what was actually withheld. You could try to apply (to the gross dividends) the lower of the two DTAs (Switzerland-ShareCountry and ETFCountry-ShareCountry). But while you will not pay too few tax, it is also not hard accounting data.
The ETFs could calculate the needed numbers (gross dividends and tax withheld) much easier. They have all the accounting entries.