Q:

PHP scheduled email alerts using preselected timezone

I've been searching alot and I can't seem to find a solution.
I have a PHP web page where I can schedule email alerts at specific times. I select the date and time of when the alert is to be sent. This is stored in MySQL table in UNIX format. I have a job that executes every 15 minutes and sends the emails if the date+time is in the past - this all works perfectly except I need to extend it for my USA colleagues. I am based in Ireland so I will need to manage the different timezones all across the US. I am planning on adding a select list of timezones that the user will have to select once they register...at least thats a start. Managing timezones is fine because I reckon all I need to do is minus the time different from the server time and then save the date time in unix format. DST is a different issue tho - does anyone have any ideas on how to overcome this? 
I have read that using UTC as the base time but even if that is the case wont I still have the same issue?
Thanks a mil! 

A:

Your times are already in UNIX timestamp format so all you need is to calculate time difference (offset) of user. For example Ireland is in UTC/UTC+1 timezone. So all you need is to do the math like james_t mentioned in his comment. If you need user from Ireland to get an email in 9:00pm UTC+1 you have to send it in 8:00pm UTC. So what's an idea?
Allow users to select timezone which means they select offset from UTC. Keep it somewhere in database (your users table or some other table, doesn't matter).
Convert offset in seconds:
$delta_time = -1 * $offset * 3600

Then calculate your trigger-time:
$trigger_time = time() + $delta_time;

Now you have time when your email sender can fire in right moment depending on user's timezone settings.
For example:
$offset = 1; // summer in Ireland
$server_time = time(); // at the moment 1350509127
$delta_time = -1 * $offset * 3600; // -3600
$trigger_time = $server_time + $delta_time; // 1350505527

All you need is to compare $trigger_time with UNIX timestamp from your database and decide to send an email (or not).
Ofc, it's not bad idea to use PHP timezones instead of pure +/- offset and stay updated when DST changes apply on certain locations.
Make some test, this is not that hard.
This is just a general idea, not complete working solution.
Update:
To calculate time-difference in seconds between any two timezones you can use something like this:
function delta_offset()
  function delta_offset($server_timezone, $user_timezone) {
    $dt = new DateTime('now', new DateTimeZone($server_timezone));
    $offset_server = $dt->getOffset();
    $dt->setTimezone(new DateTimeZone($user_timezone));
    $offset_user = $dt->getOffset();
    return $offset_user - $offset_server;
    }

To get an offset in seconds:
  $server_tz = "Europe/London";
  $user_tz = "America/New_York";
  $offset = delta_offset($server_tz, $user_tz);  // offset in sec.

Create some output:
  $dt = new DateTime('now', new DateTimeZone('UTC'));
  echo "<pre>UTC date/time: " . $dt->format('l, F jS, <b>H:i:s</b>') . "\n";
  $dt = new DateTime('now', new DateTimeZone($server_tz));
  echo "London date/time: " . $dt->format('l, F jS, <b>H:i:s</b>') . "\n";
  $dt = new DateTime('now', new DateTimeZone($user_tz));
  echo "New York date/time: " . $dt->format('l, F jS, <b>H:i:s</b>') . "\n\n";
  echo "Time difference between London (UK) and New York (USA) is: <b>$offset_h</b> ($offset s)</pre>";  

Output in browser (in moment of writing this post):
UTC date/time: Wednesday, October 17th, 22:32:27
London date/time: Wednesday, October 17th, 23:32:27
New York date/time: Wednesday, October 17th, 18:32:27

Time difference between London (UK) and New York (USA) is: -5:00 (-18000 s)

In this case offset is -5 hours (-18000 seconds) but it automatically changes if DST rules change for any of timezones given as function-arguments.
Delta-offset provides information how much earlier or later you have to send an email to user in different timezone and all you need now is to do simple +/- delta-offset with your email-sender's scheduler.
Hope this may help you to get right solution for your problem.
Update #2 - Example (theory)
Imagine this situation.
Your current server-time is X and its 7:00pm at the moment in Ireland. I live in Serbia and we have same DST rules but I’m one hour after (UTC+1/UTC+2). Difference between your and my time is +3600 seconds (1 hour).
Now you have to send an email to me in 10:00pm (it’s 9:00pm in Ireland).

Current time is X.
Delta-offset is -1 * +3600 = -3600 (delta-offset multiplied with -1).
Sending time on your location in 10:00 pm is X + 10800 (3 hours later).
Sending time  on my location in 10 pm is X + 10800 + delta-offset = X + 7200 (2 hours later).

Formula to check if actual time is equal or greater than trigger-time (sending time) is:
current_timestamp >= trigger_timestamp + delta_offset

where delta-offset from delta_offset() function must be multiplied with -1 to use in formula.
Now you can send email when you want and be sure it will be sent using user's local time (ofc, if user timezone settings are correct).
Note: Difference from this example (Serbia - Ireland = +1 hour) is different during DST changes which means that 1 hour every year we're in same timezone (0 delta-offset), and one hour we have +2 hours delta-offset. This is possible because DST changes are applied 1 hour earlier in Serbia so when our time is changed +1 you have to wait 60 minutes before same change applies to Ireland-time *then we're +2) and same thing when we bring back clock to normal time (0 difference).

