WordPress - Moment Timezone times

timezone.js

// required moment-timezone-with-data.js
String.prototype.toLocalTime = function() {
    return moment(moment.tz(this, 'YYYY-MM-DD HH:mm:ss', wpTimezone))
            .tz(moment.tz.guess())
            .format('YYYY-MM-DD HH:mm:ss');
};

String.prototype.toWPTime = function() {
    return moment(this, 'YYYY-MM-DD HH:mm:ss').tz(wpTimezone).format('YYYY-MM-DD HH:mm:ss');
};

timezone.php

<?php
/**
 * WP Timezone Settings
 *
 * @return string
 */
public static function get_wp_timezone() {
    $momentjs_tz_map = [
        'UTC-12'    => 'Etc/GMT+12',
        'UTC-11.5'  => 'Pacific/Niue',
        'UTC-11'    => 'Pacific/Pago_Pago',
        'UTC-10.5'  => 'Pacific/Honolulu',
        'UTC-10'    => 'Pacific/Honolulu',
        'UTC-9.5'   => 'Pacific/Marquesas',
        'UTC-9'     => 'America/Anchorage',
        'UTC-8.5'   => 'Pacific/Pitcairn',
        'UTC-8'     => 'America/Los_Angeles',
        'UTC-7.5'   => 'America/Edmonton',
        'UTC-7'     => 'America/Denver',
        'UTC-6.5'   => 'Pacific/Easter',
        'UTC-6'     => 'America/Chicago',
        'UTC-5.5'   => 'America/Havana',
        'UTC-5'     => 'America/New_York',
        'UTC-4.5'   => 'America/Halifax',
        'UTC-4'     => 'America/Manaus',
        'UTC-3.5'   => 'America/St_Johns',
        'UTC-3'     => 'America/Sao_Paulo',
        'UTC-2.5'   => 'Atlantic/South_Georgia',
        'UTC-2'     => 'Atlantic/South_Georgia',
        'UTC-1.5'   => 'Atlantic/Cape_Verde',
        'UTC-1'     => 'Atlantic/Azores',
        'UTC-0.5'   => 'Atlantic/Reykjavik',
        'UTC+0'     => 'Etc/UTC',
        'UTC'       => 'Etc/UTC',
        'UTC+0.5'   => 'Etc/UTC',
        'UTC+1'     => 'Europe/Madrid',
        'UTC+1.5'   => 'Europe/Belgrade',
        'UTC+2'     => 'Africa/Tripoli',
        'UTC+2.5'   => 'Asia/Amman',
        'UTC+3'     => 'Europe/Moscow',
        'UTC+3.5'   => 'Asia/Tehran',
        'UTC+4'     => 'Europe/Samara',
        'UTC+4.5'   => 'Asia/Kabul',
        'UTC+5'     => 'Asia/Karachi',
        'UTC+5.5'   => 'Asia/Kolkata',
        'UTC+5.75'  => 'Asia/Kathmandu',
        'UTC+6'     => 'Asia/Dhaka',
        'UTC+6.5'   => 'Asia/Rangoon',
        'UTC+7'     => 'Asia/Bangkok',
        'UTC+7.5'   => 'Asia/Bangkok',
        'UTC+8'     => 'Asia/Shanghai',
        'UTC+8.5'   => 'Asia/Pyongyang',
        'UTC+8.75'  => 'Australia/Eucla',
        'UTC+9'     => 'Asia/Tokyo',
        'UTC+9.5'   => 'Australia/Darwin',
        'UTC+10'    => 'Australia/Brisbane',
        'UTC+10.5'  => 'Australia/Adelaide',
        'UTC+11'    => 'Australia/Melbourne',
        'UTC+11.5'  => 'Pacific/Norfolk',
        'UTC+12'    => 'Asia/Anadyr',
        'UTC+12.75' => 'Asia/Anadyr',
        'UTC+13'    => 'Pacific/Fiji',
        'UTC+13.75' => 'Pacific/Chatham',
        'UTC+14'    => 'Pacific/Tongatapu',
    ];

    $current_offset = get_option('gmt_offset');
    $tzstring = get_option('timezone_string');

    // Remove old Etc mappings. Fallback to gmt_offset.
    if ( false !== strpos( $tzstring, 'Etc/GMT' ) ) {
        $tzstring = '';
    }

    if ( empty( $tzstring ) ) { // Create a UTC+- zone if no timezone string exists
        if ( 0 == $current_offset ) {
            $tzstring = 'UTC+0';
        } elseif ($current_offset < 0) {
            $tzstring = 'UTC' . $current_offset;
        } else {
            $tzstring = 'UTC+' . $current_offset;
        }

    }

    if ( array_key_exists( $tzstring , $momentjs_tz_map ) ) {
        $tzstring = $momentjs_tz_map[ $tzstring ];
    }

    return $tzstring;
}