Stock split question, I.e. AMZN

I've held AMZN forever. When it splits, it's just “funny math”, right?, I'm no better off, no worse off. In a 20-for-1 stock split, I have 20 times as many stocks as before the split, each worth 1/20th of what they were worth. Got it.

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But — for USA IRS short-term vs. long-term capital gains calculations of any subsequent sales — what is the purchase date of any new stocks? Split date?, or my original purchase date of the pre-split stocks?

I.e. am I given 19 new stocks for each stock I held? Or are my existing stocks each replaced with 20 with purchase date details of the original stock? If the former, is my new holding date the date I'm given the 19 new stock (I.e. the date they split, the date I'm “given 19 new stocks”), … or backdated to when I purchased the original?

I'm pretty sure the purchase date of the new stock can't be the stock-split date? Surely? But I can't find anything definitive.

Thanks,