Source

File: wp-includes/functions.php

function wp_timezone_string() {
	$timezone_string = get_option( 'timezone_string' );

	if ( $timezone_string ) {
		return $timezone_string;
	}

	$offset  = (float) get_option( 'gmt_offset' );
	$hours   = (int) $offset;
	$minutes = ( $offset - $hours );

	$sign      = ( $offset < 0 ) ? '-' : '+';
	$abs_hour  = abs( $hours );
	$abs_mins  = abs( $minutes * 60 );
	$tz_offset = sprintf( '%s%02d:%02d', $sign, $abs_hour, $abs_mins );

	return $tz_offset;
}