Server Time - driving me nuts :-) - php

Question

Okay so I have a server in Denver with a user in New Zealand. I know everything about the user (timezone etc) and through the program they request something to happen in advance - let's say at 11:30am on August 5th 2013. I have a CRON job that runs every 15 minutes and asks the database if any requests are pending for the next 15 minute period, but how do I convert their stored time to the servers equivalent.
I set the default timezone for calculations: date_default_timezone_set('America/Denver')
I take the time now on the server and turn it into epoch: strtotime(date('Y-m-d H:i:s'))
I add the 15 minutes to create a range: $forward15 = strtotime('now +15 minutes')
I get the user chosen date from the database (and their timezone): 2013-08-05 11:30:00
Now what? If I convert that into epoch, it'll just be the servers version of that date.

Answer 1

NEW SOLUTION SEE BELOW!
If you know the timezone you can simply "add" it to your time.
For example:
server time: 01/01/01 00:00
the time the user wants: 01/01/01 01:00
the timezone of the user: GMT - 5
Just get the time (01/01/01 01:00) and add +5 => 01/01/01 06:00
So: your script needs to be executed at 01/01/01 06:00
(convert to timestamp where needed)
Added a little php to demonstrate
<?php
$servertime = time(); //timestamp of 01/01/01 00:00
$usertime = "01/01/01 06:00";//database
$userUTC = "-5";//database
strreplace($userUTC, "-", "+";
$replacetext = $userUTC . " days";
$usertime = strtotime($replacetext, $usertime);//now usertime is in your local timezone
$crontime = date("d/m/Y H:i");//the time you want the script to be executed
?>
I'm just assuming that the timezone is saved as "-5" for example and not Europe/Amsterdam Just tell me if i'm wrong.
edit 14:37
This could be a even better solution i think!
<?php
$usertime = "01/01/01 06:00";
$userUTC = "-5";
$userdate = $usertime . " " . $userUTC;
$usertimestamp = strtotime($userdate);//now you have the timestamp with correct timezone
$crontime = date("d/m/Y H:i", $usertimestamp);//formatted to the right date
echo $crontime;
?>
Edit: 25-07-2013 14:26
New solution to suit your database:
<?php
$usertime = "01/01/01 06:00";
$userUTC = "Pacific/Auckland";//get from database
$userdate = $usertime . " " . $userUTC;
$usertimestamp = strtotime($userdate);//now you have the timestamp with correct timezone
$crontime = date("d/m/Y H:i", $usertimestamp);//formatted to the right date
echo $crontime;
?>

Answer 2

the server is in which GMT time zone here is extremely easy way to get time and date for any time zone. This is done with time() and gmdate() function. gmdate() function normally give us GMT time but by doing a trick with time() function we can get GMT+N or GMT-N means we can get time for any GMT time zone.
For example you have to get time for GMT+5 we will do it like following
<?php
$offset=5*60*60; //converting 5 hours to seconds.
$dateFormat="d-m-Y H:i";
$timeNdate=gmdate($dateFormat, time()+$offset);
?>
Now if you have to get the time which is GMT-5 now we will just subtract the offset from the time() instead of adding into time like in following example we are getting time for GMT-4
<?php
$offset=4*60*60; //converting 5 hours to seconds.
$dateFormat="d-m-Y H:i";
$timeNdate=gmdate($dateFormat, time()-$offset);
?>