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.

Published by Bramus!

Bramus is a frontend web developer from Belgium, working as a Chrome Developer Relations Engineer at Google. From the moment he discovered view-source at the age of 14 (way back in 1997), he fell in love with the web and has been tinkering with it ever since (more …)

Unless noted otherwise, the contents of this post are licensed under the Creative Commons Attribution 4.0 License and code samples are licensed under the MIT License

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