Here's a suggestion:
Add a timezone field for your members. Make it so that they can set their own timezones during member creation.
If an event is 7:00pm pacific time, a member who chose an eastern time zone would read the pacific time + 3 hours, which is 10:00pm.
Here's how I'd go about it.
In the registration form (where you set the username, password, all that good stuff) make a select field for timezone. Something like this:
Then in your php code, do something like:
$query = mysql_query("select * from userTable where username='bob");
$result = mysql_fetch_array($query));
/* No need to alter time if timezone is 1 (pacific)
if ($result[timezone] == 2) $time + 1;
elseif ($result[timezone] == 3) $time + 2;
/* ... Etc, etc ... */
Please do not plain copy and paste this code for it will not work. This is just a demo.. or to better put it, a "pseudocode" to place you on the right track