PHP: Convert a Geolocation (Latitude / Longitude Coordinates) to a Timezone identifier

Part of a PHP project I’m working contains a list of sites/buildings. For each site/building we monitor some data, for example its energy usage.

We decided that we wanted to generate a daily/weekly/monthly reports of the data, by aggregating the datapoints. As our sites/buildings are spread across the globe – and thus timezones – we can’t simply select data between 00:00:00 UTC and 23:59:59 UTC but have to use its geographical location’s “day window” to do our calculations.

Unfortunately we don’t didn’t store the timezone for a site/building, but since we do keep track of its geographical location – using a WGS84 latitude-longitude pair – it should be possible to derive its timezone, right?

Right! On StackOverflow I found this little snippet that does the job:

function get_nearest_timezone($cur_lat, $cur_long, $country_code = '') {
    $timezone_ids = ($country_code) ? DateTimeZone::listIdentifiers(DateTimeZone::PER_COUNTRY, $country_code)
                                    : DateTimeZone::listIdentifiers();

    if($timezone_ids && is_array($timezone_ids) && isset($timezone_ids[0])) {

        $time_zone = '';
        $tz_distance = 0;

        //only one identifier?
        if (count($timezone_ids) == 1) {
            $time_zone = $timezone_ids[0];
        } else {

            foreach($timezone_ids as $timezone_id) {
                $timezone = new DateTimeZone($timezone_id);
                $location = $timezone->getLocation();
                $tz_lat   = $location['latitude'];
                $tz_long  = $location['longitude'];

                $theta    = $cur_long - $tz_long;
                $distance = (sin(deg2rad($cur_lat)) * sin(deg2rad($tz_lat))) 
                + (cos(deg2rad($cur_lat)) * cos(deg2rad($tz_lat)) * cos(deg2rad($theta)));
                $distance = acos($distance);
                $distance = abs(rad2deg($distance));
                // echo '<br />'.$timezone_id.' '.$distance; 

                if (!$time_zone || $tz_distance > $distance) {
                    $time_zone   = $timezone_id;
                    $tz_distance = $distance;
                } 

            }
        }
        return  $time_zone;
    }
    return 'unknown';
}

Usage is as follows:

// Timezone for one NY coordinate
echo get_nearest_timezone(40.772222,-74.164581);
// ~> America/New_York

// Timezone for one Belgian coordinate
echo get_nearest_timezone(51.0162167, 3.7338451);
// ~> Europe/Brussels

// More faster and accurate if you can pass the country code 
echo get_nearest_timezone(40.772222, -74.164581, 'US');
// ~> America/New_York

With this timezone identifier now being available, we can include it in our queries and generate our daily/weekly/monthly reports 🙂

🍻 Here’s to copying-and-pasting from StackOverflow!

On a related note: Falsehoods programmers believe about time and time zones is worth a read, especially if you’ve already dealt with time and timezones.

Why Denmark Is .17 Seconds Behind The World

Measuring time is a complicated thing. Computers, banks, and stock markets in Denmark all use UTC, the international standard: but according to the law, they shouldn’t.

🤓 Also see Falsehoods programmers believe about time and time zones

How to Perform Calendar Calculations in Your Head

In case you want to impress your family over Christmas’/New Year’s Dinner:

A so-called calendrical savant (or calendar savant) is someone who despite their intellectual disability (typically autism) can name the day of the week of a given date, or visa versa in a few seconds or even a tenth of a second (Kennedy & Squire, 2007).

While extremely impressive to behold, calendar calculations are actually very simple to perform and can be learned in less than 30 minutes. This short article will teach you how.

How to Perform Calendar Calculations in Your Head →

💵 This article is stuck behind Medium’s metered paywall … open the link in an incognito window to bypass that limit.

spatie/test-time – A PHP package to control the flow of time

Freek has created spatie/test-time, a package to easily freeze/rewind/advance time in PHP.

Imagine you’re building that your app can notify your user, but you don’t want to send more than one notification in a timeframe of five seconds. How are you going to test the time aspect? Do you have to create a test that takes five minutes?

Luckily the answer is “no”. If you’re using the popular Carbon library, you can set the value that the library considers “now”

The package provides a few convenience methods around Carbon’s setTestNow method:

// time will not progress anymore
TestTime::freeze();

// Advance one minute
TestTime::addMinute();

// Rewind 5 hours
TestTime::subHours(5);

💁‍♂️ If you are not using Carbon you can reside to the php-timecop PHP extension that does the job (and affects \DateTime).

A PHP package to control the flow of time →
spatie/test-time (GitHub) →

Symfony Form Validation: Validating a date range

One of the (Symfony based) PHP projects I’m working on contains a form which allows the user to generate video clips from CCTV footage. To do this the user can enter a start and stop DateTime. For this to work the submitted input data is then checked: both start and stop must be dates, and the stop date must be set to a point in time after the start date.

Symfony’s DateTime Constraint can make sure both entries are DateTime instances. To check whether that the end date is after the begin date, one can use the Callback Constraint. Injected into that callback is a ExecutionContextInterface by which you can access the form, and thus other form params.

Here’s an example with the inputs start and stop:

use Symfony\Component\Validator\Constraints;
use Symfony\Component\Validator\Context\ExecutionContextInterface;

// …

$builder
    ->add('start', 'datetime', 
        'constraints' => [
            new Constraints\NotBlank(),
            new Constraints\DateTime(),
        ],
    ])
    ->add('stop', 'datetime', [
        'constraints' => [
            new Constraints\NotBlank(),
            new Constraints\DateTime(),
            new Constraints\Callback(function($object, ExecutionContextInterface $context) {
                $start = $context->getRoot()->getData()['start'];
                $stop = $object;

                if (is_a($start, \DateTime::class) && is_a($stop, \DateTime::class)) {
                    if ($stop->format('U') - $start->format('U') < 0) {
                        $context
                            ->buildViolation('Stop must be after start')
                            ->addViolation();
                    }
                }
            }),
        ],
    ]);

If you want to have a minimum duration between both start and stop, you can change the number 0 in the snippet above to any number of seconds.

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php-timecop – A PHP extension providing “time travel” capabilities

Now this looks handy, especially when you’re writing/testing code that’s dependent on the system’s time:

php-timecop is a PHP extension providing “time travel” and “time freezing” capabilities

  • Freeze time to a specific point.
  • Travel back to a specific point in time, but allow time to continue moving forward from there.
  • Scale time by a given scaling factor that will cause time to move at an accelerated pace.
  • Override time-related PHP stock functions and methods, which supports freezing or traveling time.
var_dump(date("Y-m-d")); // todays date
timecop_freeze(0);
var_dump(gmdate("Y-m-d H:i:s")); // string(19) "1970-01-01 00:00:00"
var_dump(strtotime("+100000 sec")); // int(100000)

php-timecop →
Stub PHP date() and the crew with php-timecop →

Carbon – A simple PHP API extension for DateTime.

$carbon = new Carbon('first day of next week');

if ($carbon->isWeekend()) {
    echo 'Party!';
}

echo $carbon->addYear()->diffForHumans(); // 'in 1 year'

Carbon is just a class which is designed to be used instead of DateTime. Due to extending DateTime, all DateTime methods are available to users of Carbon. Additionally, it implements a __toString method, allowing users to put it in place of string representations of date and time as well.

Installation possible using Composer, of course:

composer require nesbot/carbon

Carbon →
Carbon – Date and Time the Right Way →

Falsehoods programmers believe about time and time zones

  • Every day has 24 hours
  • Every day without DST changes is 86400 (60 * 60 * 24) seconds long
  • Every day in UTC is 86400 (60 * 60 * 24) seconds long
  • Week one of a year starts in January every year
  • If I know what time zone someone is in and they just tell me the date and local time, I can always use software to find out what time that is in UTC
  • DST always sets the clock back and forth by exactly one hour
  • Countries that observe DST begin observing DST in the first half of the year and end observing DST in the last half of the year
  • …

Be sure to click through and read a few more falsehoods, along with their explanations.

Falsehoods programmers believe about time and time zones →