How come the number of blocks left is Unknown should I be concerned?

I'm using bitcoin-qt on Linux to sync up with the blockchain, or at least that’s what I think it’s doing. Before, it used to show how many blocks were left, but now since I launched the program, it just says "Unknown. Syncing Headers (870924, 100.0%)..." The "Last block time" keeps moving forward; as I’m writing this, it says "Tue Apr 4 13:00:54 2023". Also, I see these other stats: Progress at 72.64%, Progress rising 0.28% per hour, Estimated time left to finish syncing: Unknown... Not sure if I should be worried since it seems like the tool has lost track of the block count, or is this just something that happens sometimes?

Running bitcoin core on any OS will definitely catch up (sync) with the blockchain. So you are correct that that's what it does. It looks like an issue with calculating the time that is required until fully synced. I wouldn't worry, considering that the number of blocks left keeps decreasing. What hardware are you using? SSD or HDD? Can you also post a screenshot or log of the output of the bitcoin-cli getblockchaininfo command?

Thanks for the quick reply. Just as I wanted to open the cli, the program started to report the number of blocks left as I had known it - so everything seems good now. For (possibly future) info, this was happening on an HDD.

Since you mention Bitcoin-qt you might be using the old version of Bitcoin core? Why not tell us the version of your bitcoin-qt? If you are using old version, I suggest to update it first to the most recent version and test it again if it would sync properly. You can download the latest version from here https://bitcoincore.org/en/releases/28.0/ Make sure before performing to upgrade it to the latest version you have a copy of a backup wallet.dat to a safe place for future recovery if things are going bad after the update.

This was probably some connection issues because your node should be able to quickly figure out how many blocks there are to download. It does that by first syncing its headers with the peers it connects to and the entire block headers file is only 66 MB and if your node has been running before, the data it needs to download is even smaller (80 byte per block, so 1 day worth of block headers is 11-12 KB). Meaning since the data is small, the only reason why your node would hang is if it couldn't connect or maybe the other peer was not responsive so it needed to time-out and connect to another peer.

If unsure, you can always count on the logs. To check it, open your bitcoin data directory and find debug.log file, open it as a text file and look for related lines or errors. If there's at least one, report it here together with your settings so we can suggest a solution.

How much RAM do you have? Syncing Bitcoin Core only gets slower due to the growth of your chainstate directory. I've done it on HDD with 32 GB RAM, and that's doable. But with 8 GB RAM and SSD, the SSD is already the limitation. If you're low on RAM and use HDD, you may want to reconsider and use Electrum instead. Or upgrade your PC. It's been years since I tried, but I remember starting Bitcoin Core with chainstate on HDD took minutes of looking at a "frozen" window. There is no "quickly".

Bitcoin Core can't calculate the number of blocks left until it fetches all the block headers. Which it seems to have already done. In my opinion, as long as the progress is continuously increasing, it means the node is syncing fine and that you don't have to worry about the lack of an ETA. The last block time becoming more and more recent is also normal.