Forum Moderators: open
There's no HTML tag that will do this. Javascript is the most common approach on the client side, but it depends on the user's computer clock being accurate. The web is BULGING with scripts that do this. I'd suggest you start with this resource: a search engine that searches all the major javascript resources on the web:
[javascript-2.com...]
There are also cgi/perl solutions that are outside the scope of this forum, and I'm sure a host of other server-side solutions as well. Any search engine should return to you a big pile of places to look for this kind of script.
I hope you will understand this decision -- that I'm locking this thread from any further conversation. The topic is just too big a temptation for people to link to their own site or drop a mention of their friend's script, and we don't allow that.
As I already mentioned, the web is full of these solutions, but there is nothing in HTML itself that provides this functionality.