There is no install procedure. The files just need to reside in the same folder on a webserver that has PHP installed, and then the two files:
You can use the browser's zooming capabilities to enlarge the clocks once they are running (if screen space permits).
The download of the webpages requires an internet connection, but once the clock page is setup and displayed, the connection can be terminated (or lost) with no consequences - except that you no longer can change the setup values.
You can in fact use the setup page and save the values, open the clock page and then use your browser's capability to save the page locally as pure HTML. This page can then be opened locally and run with no internet connection. The only drawback is that you cannot change the values (except by going through the setup and save procedure once again).
It should be noticed that the setup values are saved to a file with fixed filename. This means that a new user will override the values of the previous one. This does not affect a running clock (or a locally saved one), but a reload page would change the clock.