oh, a fourth way which is sort of derived from method 1, as you indicated, automatic updating after page loaded, I guess that was probably the question to begin with...
If you want continuesly updating real-time values you would need to use a java-applet or something like that, using a socket connection to your server (Java applet security would strict the connection to the server it was downloaded from).
For reloading/updating partial content you can use JavaScript and layers, for a text-only clock javascript should be fine.
As for a clock/time/countdown there are already some of those out there, perhaps
this page has one you like, otherwise just search around...
As for temperature, it really depends on where the value is coming from, you could potentially put a sensing device on your howm machine or server and upload/push the value to your webserver every X minutes..
My first mentioned "method" of "real-time" was to get the value from the sensing device/service at the time of page requested, could be done either server side or client side depending somewhat on where you are getting the values from.
A scheduled "pull" method would be serverside on your site, a cron-job, running a script every X minutes that fetches the value from some service/device and stores in a file on your site..
A scheduled "push" method would be 3-tiered-serverside, the device/system that measures will upload the value to your server, which in turn serves the web pages/values to the world. This would be a typical approach if you have a sensing device at home, much like many web-cam softwares can do, upload every X minutes..