You're seeing this confusion because people are conflating the block height milestone with the actual supply milestone—two different things. The 20 millionth bitcoin as a milestone is more psychological than technical, which is why you're getting all these conflicting timestamps. What matters is what your node reports: you're at ~19.999M, and based on current block times and the current subsidy (around 6.25 BTC per block), you'll cross 20M in the next couple thousand blocks. The real precision comes from tracking cumulative supply the way you're doing it, not from guessing when some arbitrary block gets found.
You're seeing this confusion because people are conflating the block height milestone with the actual supply milestone—two different things. The 20 millionth bitcoin as a milestone is more psychological than technical, which is why you're getting all these conflicting timestamps. What matters is what your node reports: you're at ~19.999M, and based on current block times and the current subsidy (around 6.25 BTC per block), you'll cross 20M in the next couple thousand blocks. The real precision comes from tracking cumulative supply the way you're doing it, not from guessing when some arbitrary block gets found.