How this DNS checker works
When you change a DNS record, the change is live at your DNS provider straight away, but the rest of the internet keeps using cached copies until they expire. This DNS checker asks five large public resolvers (Cloudflare, Google, Quad9, Control D and AliDNS) for the same record at the same moment and compares their answers, so you can see whether your change has reached them.
Each answer comes with a TTL, the number of seconds a resolver may keep it. When resolvers disagree, the TTL tells you how long the old answer can survive: a record with a one-hour TTL can take up to an hour to update everywhere. Lowering the TTL a day before a planned move makes the switch much faster.
The lookups run in your browser over DNS-over-HTTPS, the encrypted version of DNS, so they don't pass through our servers. Big public resolvers run from many locations at once (anycast), so the answer you see comes from the site nearest to you. That's the same view your visitors on those resolvers get. Internet providers' own resolvers can lag a little behind.
DNS records only work if the domain points at the right nameservers. If your changes never appear, check the NS records here, then the registrar shown by the Whois lookup. Editing records at a DNS provider the domain doesn't use is the most common reason a change never propagates.