https://news.ycombinator.com/item?id=21268584 it shows "4 hour ago"(by time I posted this question)
API: https://hacker-news.firebaseio.com/v0/item/21268584.json unix time:1571218522, and it is 3 days ago.
It is not ideal.
You can see https://news.ycombinator.com/item?id=21268584 is "5 hours ago", but from my tool(https://hn.okphp.com/?k=digital,postgres ) it is "3 days ago"(my tool is using official api timestamp)