Creating timestamp for 5/13/2014 # 3pm - php

Question

I have an application that needs to send a UTC timestamp in order for it to work correctly. In my application a user can have any number of timezones. So if they pick 3pm and their timezone is America/New_York, it is a different 3pm than if it was America/Chicago.
I need to figure out a way to change the date into the right UTC timestamp. I know I can use date_default_timezone_set("UTC")...but I don't think will work correctly.
I think I need to calculate a difference between UTC and regular timezone, but I am not sure. Any advice is welcomes.
date_default_timezone_set("UTC");
echo strtotime('5/13/2014 3:00 PM');
1399993200
date_default_timezone_set("America/New_York");
echo strtotime('5/13/2014 3:00 PM');
1400007600
As you can tell these 2 values are different.
EDIT: Here is what my code looks like. It doesn't seem to work correctly as the application doesn't show the event in the right time.
$previous_timezone = date_default_timezone_get();
date_default_timezone_set("UTC");
$aceroute_schedule = $this->sale_lib->get_send_to_aceroute_schedule();
if (($start_time = strtotime($aceroute_schedule['aceroute_schedule_date']. ' '.$aceroute_schedule['aceroute_schedule_time_start'])) !== FALSE)
{
//Append 000 as as string for 32 bit systems
$start_epoch = $start_time.'000';
$end_epoch = strtotime('+ '.$aceroute_schedule['aceroute_duration'].' minutes', $start_time).'000';
}
else //Default to current time + 1 hour
{
//Append 000 as as string for 32 bit systems
$start_epoch = time().'000';
$end_epoch = strtotime('+1 hour', time()).'000';
}
$event->start_epoch = $start_epoch;
$event->end_epoch = $end_epoch;

Answer 1

Update:
This will now create a DateTime object in the user's DateTimeZone ('America/New_York'). And then it will set that object's timezone to UTC. To get the timestamp (or other string representations of date), use ::format().
# Create NY date
$NY = new DateTimeZone("America/New_York");
$NYdate = new DateTime('5/13/2014 3:00 PM', $NY);
# Set timezone to UTC
$UTC = new DateTimeZone("UTC");
$UTCdate = $NYdate->setTimezone($UTC);
# Get timestamp (PHP 5.2 compatible)
$timezone = $UTCdate->format('U');
var_dump($timezone); // a string containing UNIX timestamp
First I create 2 DateTime objects based off of their respective DateTimeZone objects. Then we can either use OOP ::diff() to get another object containing information about the time difference. Or we can use simple integers representing the difference in seconds from ::getTimestamp.
$date = '5/13/2014 3:00 PM';
# Create NY date
$NY = new DateTimeZone("America/New_York");
$NYdate = new DateTime($date, $NY);
# Create UTC date
$UTC = new DateTimeZone("UTC");
$UTCdate = new DateTime($date, $UTC);
# Find difference object
$diff = $NYdate->diff($UTCdate);
var_dump($diff); // a DateInterval object containing time difference info
# Find difference in seconds
$diff = $NYdate->getTimestamp() - $UTCdate->getTimestamp();
var_dump($diff); // an integer containing time difference in seconds
Links:
DateTimeZone
DateTime
DateInterval

Answer 2

Example in http://www.php.net/manual/en/datetime.settimezone.php
$date = new DateTime('2000-01-01', new DateTimeZone('Pacific/Nauru'));
echo $date->format('Y-m-d H:i:sP') . "\n";
$date->setTimezone(new DateTimeZone('Pacific/Chatham'));
echo $date->format('Y-m-d H:i:sP') . "\n";
The first line creates a DateTIme object, using the timezone Pacific/Nauru.
You can then change the timezone using setTimezone as shown in line 4, and the output will be modified accordingly.
note: the default timezone (if you don't specify it in the 2nd parameter in line 1) is the one set in your php.ini file, which you can modify (at runtime) with date_default_timezone_set("America/New_York")
note2: the 1st parameter in line 1, is equivalent to the 1st parameter of the strtotime function.
note3: the format method takes the same format parameter as date (http://www.php.net/manual/en/function.date.php)