Is there a way to force the visitor's browser to refresh all content?

Let’s say I want to make sure that the visitor’s browser
grabs my latest copy of the CSS file, JavaScript files even Image files…
the whole schebang.

Does this do that:
(because the date has already passed)

<meta http-equiv="expires" content="Fri, 31 Dec 1999 12:00:00 GMT" />

And if so is there any restrictions on the date AND/OR date format used?
For example what if its not a valid calendar date (31 Feb for example).

According to this article, the expires tag is useless and is ignored.
Meta Expires Tag Reviewed

Apparently though you can set the HTTP header to expire, which seems to affect whether pages retrieve the latest copy.

this can be done by automatically adding a rotated ID to the files in question in your site code, or more cleanly by expiring the HTTP header (webservers can do that - we do it with nginx, not sure how the command syntax works with Apache)

The most common way I’ve seen it done is, like oinari says, by adding some form of id to the url of the file you’re referencing, and just changing the id every time you update the file.

For example, for version 1 of your stylesheet

<link rel="stylesheet" href="styles.css?v1" />

If you update your stylesheet, you’d simply increment the version number

<link rel="stylesheet" href="styles.css?v2" />

This would force the browser to download the updated stylesheet.

Here are two other useful links:

Can We Prevent CSS Caching? | CSS-Tricks

Particletree » Automatically Version Your CSS and JavaScript Files

I actually came up with an idea nearly identical to ParticleTree, though I just apply the filemtime as a “ver” parameter: mypage?ver=12345678. Even parameter changes cause it to be redownloaded, so it works really well (with no rewrites required).

We also have files stored on another server which I want to do the same thing with, but filemtime doesn’t work over http. In those cases, you can fopen() the file, then use stream_get_meta() to get it’s meta data and fine the “Last Modified” meta data.