PHP将工作日和时间（小时）转换为GMT表示

Question

I have a web application that allows users to select to have a certain operation performed at a specific hour on a specific weekday (ex: 7pm Friday) -- recurring every weekday. My users are located in different timezones. Therefore I would like to store the day/hour in one time zone format so that my cron job which runs hourly can query my database and quickly grab all the rows that represent tasks to be performed that hour. What's the best way to accomplish this?

Basically I need to know how to take Friday 7:00pm in Boston, MA or Saturday 3:00pm San Francisco, CA and convert both to GMT (and store in my DB). That way when I run my cron job once an hour I know exactly what value to query the database for given what GMT day/hour it currently is.

EDIT 1 Through playing around I came up with something:

date_default_timezone_set('America/New_York');
$time = strtotime('Friday 8:00pm');

echo date('w : G', $time);
echo '<br>';

date_default_timezone_set('GMT');
$date = date('w : G', $time);

echo $time;
echo '<br>';
echo $date;

The output is:

5 : 20
1331942400
6 : 0

Is this basically the right track?

EDIT 2 Useful detail, I'm actually lucky enough to have my user's timezone information stored as one of these: http://www.php.net/manual/en/timezones.php

Answer 1

If you store the users time zone already, you can add the GMT offset to the time to get the GMT date.

I'm located in Colorado which is currently -7 GMT:

$offset = -7;
$gmt = ($offset * 60 * 60) + time();

would give the the unix timestamp for the GMT timezone.

Then make sure your server has GMT as the default or set timezone, you can set the timezone using:

date_default_timezone_set('GMT');