You can use "View page source" on the non-mobile site and search (Ctrl-F) for the first occurrence of data-time=" to get the "unix timestamp" (number of seconds since 1 Jan 1970).
view-source:https://twitter.com/realdonaldtrump/status/1039235839333679105
For posts here, you can use your browser's developer tools to select the "time" element of a post, which has a unixtime property with a "unix timestamp" that is directly comparable to those from twatter.