Hottest day recorded, likely since 1871 (1), but actually we don't know because the article doesn't offer that information. The Earth has been around much longer though.
(1): https://data.noaa.gov/onestop/collections/details/55bedd68-b...
(1): https://data.noaa.gov/onestop/collections/details/55bedd68-b...
Why does it matter how long the earth has been around? We're not talking about the hottest day of all time (remember that for a long time the earth had a molten crust), we're specifically talking about the hottest recorded day. It sounds like you're putting it as a gotcha when it's just the topic.