Set the local users timezone offset to a cookie with Javascript and pull it with PHP to set the default timezone

gistfile1.js

/**
 * Set the local time offset to display time values in the users local time
 */
function setTimeOffset() {
	// Create all the dates necessary
	var now = later = d1 = d2 = new Date(),
		set = { 'offset': now.getTimezoneOffset(), 'dst': 0 }
	// Set time for how long the cookie should be saved - 1 year
	later.setTime(now.getTime() + 365 * 24 * 60 * 60 * 1000);
	// Date one is set to January 1st of this year
	// Guaranteed not to be in DST for northern hemisphere and guaranteed to be in DST for southern hemisphere (If DST exists on client PC)
	d1.setDate(1);
	d1.setMonth(1);
	// Date two is set to July 1st of this year
	// Guaranteed to be in DST for northern hemisphere and guaranteed not to be in DST for southern hemisphere (If DST exists on client PC)
	d2.setDate(1);
	d2.setMonth(7);

	// DST exists for this time zone – check if it is currently active
	if ( parseInt( d1.getTimezoneOffset() ) != parseInt( d2.getTimezoneOffset() ) ) { 
		// Find out if we are on northern or southern hemisphere - Hemisphere is positive for northern, and negative for southern
		var hemisphere = parseInt( d1.getTimezoneOffset() ) - parseInt( d2.getTimezoneOffset() );

		if (
			( hemisphere > 0 && parseInt( d1.getTimezoneOffset() ) == parseInt( now.getTimezoneOffset() ) ) ||
			( hemisphere < 0 && parseInt( d2.getTimezoneOffset() ) == parseInt( now.getTimezoneOffset() ) ) 
		) {
			// DST is active right now
			set.dst = 1;
		}
	}
	document.cookie ='time_zone='+ JSON.stringify(set) +'; expires='+ later +'; path=/';
}

// Trigger the time offset cookie creation
setTimeOffset();

gistfile2.php

// Check for the time_zone cookie, if set then reset the default timezone
if ( isset($_COOKIE['time_zone']) ) {
	$time_zone = json_decode( $_COOKIE['time_zone'], true );
	// Make sure there's an available abbreviation before setting the timezone
	if ( timezone_name_from_abbr( '', -$time_zone['offset']*60, $time_zone['dst'] ) ) {
		date_default_timezone_set( timezone_name_from_abbr( '', -$time_zone['offset']*60, $time_zone['dst'] ) );
	}
}