Timezone help needed

[Anon]

I am a novice developer trying to put together a scheduling system that
can support multiple timezones. My program is mainly in php with mySQL
backend. What I want to do is allow people to schedule an event in
their local time, and store it in gm time. Then, when other people see
the event displayed, it appears in THEIR local time. If possible, I
would also like to determine some sort of region or TZ name so I could
group users by timezone and do stuff like list all the users in a
particular region.

I've been able to get the offset from the client machine with Javascript
using:

echo "<SCRIPT LANGUAGE=\"JavaScript\">\n";
echo "var today = new Date()\n";
echo "document.writeln(today.getTimezoneOffset()/60)\n";
echo "</SCRIPT> : \n";

But I need to get the value of this into a var I can use in php. If I
could store the value in a variable, say $timeOffset, I could display
local time like:

echo date( "M d Y H:i:s",
mktime(gmdate("H")-$timeOffset,gmdate("i"),gmdate("s"),gmdate("m"),gmdate("d"),gmdate("Y"))
);

How to figure out the particular region and such is totally beyond me. Any help would be greatly appreciated!

[Anon]

Your best bet is to get Javascript to write a hidden form field on some kind of login form.

<script language="JavaScript1.2"><!--
x = new Date();
document.write("<input type='hidden' name='timezone_offset' value='" + x.getTimezoneOffset()*-60 + "'>");
// -->
</script>

The above example multiplies by -60 to produce a seconds offset from UTC, which is most useful for adding and subtracting from UNIX timestamps

Once this form has been submitted, store their timezone offset in a session variable.