Datetime conversion functions for WordPress using the ISO8601 standard.

wp-iso8601-datetime.php

<?php
/**
 * Why does this exist? http://xkcd.com/1179/ also because it makes sense to have a standard way in WordPress to work with
 * dates and times so that plugin developers can create plugins that will work in any timezone worldwide.
 * 
 * WordPress sets the default timezone during execution to UTC, reguardless of what the server is set to.
 * This means that date( 'FORMAT' ) will always give you the current time in UTC, not the timezone you have set
 * in the WP admin settings. Use date( 'FORMAT', current_time( 'timestamp' ) ) instead to get WP local time. Likewise,
 * use current_time( 'timestamp' ) instead of time() for the local time.
 */

/**
 * Changes the timezone on an ISO 8601 formatted datetime string to GMT
 *
 * @param string $date_string ISO 8601 formatted date string (e.g. 2012-12-21T13:52-05:00 would be Dec. 21, 2012, at 1:52PM EST)
 * @return string Returns ISO 8601 formatted date string in GMT/UTC time (+00:00 timezone)
 */
function iso8601_to_gmt( $date_string ) {
    // Split the datetime into individual items
    preg_match( '#([0-9]{4})-?([0-9]{2})-?([0-9]{2})T([0-9]{2}):([0-9]{2}):([0-9]{2})(Z|[\+|\-][0-9]{2}:?[0-9]{2}?){0,1}#', $date_string, $date_bits );
    if ( ! empty( $date_bits[7] ) ) { // we have a timezone, so let's compute an offset
        $date_bits[7] = str_replace( ':', '', $date_bits[7] ); // Make sure this is in the right format for the iso8601_timezone_to_offset() function ( no colon ( : ) )
        $offset = iso8601_timezone_to_offset( $date_bits[7] );
    } else { // we don't have a timezone, so we assume user local timezone (not server's!)
        $offset = HOUR_IN_SECONDS * get_option( 'gmt_offset' );
    }
    $timestamp = gmmktime( $date_bits[4], $date_bits[5], $date_bits[6], $date_bits[2], $date_bits[3], $date_bits[1] );
    $timestamp -= $offset;
    return gmdate( 'c', $timestamp );
}

/**
 * Changes an ISO 8601 formatted date from GMT/UTC (+00:00) to WordPress' set time or the timezone supplied.
 *
 * @param string $date_string ISO 8601 formatted datetime in GMT/UTC
 * @param string $format The format you want the date to be returned in, defaults to ISO 8601. See http://php.net/manual/en/function.date.php for formatting options
 * @param string|null $tzID The timezone identifier of the timezone to convert to, see http://php.net/manual/en/timezones.php, defaults to what is set in the WP admin
 * @return string Returns formatted date string in either WordPress' local time or the specified offset
 */
function iso8601_gmt_to_local( $date_string, $format = 'c', $tzID = null ) {
    if ( null === $tzID ) {
        $tzID = get_option( 'timezone_string' );
    }
    if ( null === $tzID ) {
        $offset = ( get_option( 'gmt_offset' ) * -1 );
        $tzID = 'Etc/GMT' . ( ( 0 > $offset ) ? $offset : '+' . $offset ); // This line is here for compatibility reasons
    }
    $tz = new DateTimeZone( $tzID );
    $date = DateTime::createFromFormat( 'U', strtotime( $date_string ) );
    $date->setTimezone( $tz );
    return $date->format( $format );
}

/**
 * Changes an ISO 8601 formatted date to WordPress' set time or the timezone supplied.
 *
 * @param string $date_string ISO 8601 formatted datetime
 * @param string $format The format you want the date to be returned in, defaults to ISO 8601. See http://php.net/manual/en/function.date.php for formatting options
 * @param string|null $tzID The timezone identifier of the timezone to convert to, see http://php.net/manual/en/timezones.php, defaults to what is set in the WP admin
 * @return string Returns formatted date string in either WordPress' local time or the specified offset
 */
function iso8601_to_local( $date_string, $format = 'c', $tzID = null ) {
    return iso8601_gmt_to_local( iso8601_to_gmt( $date_string ), $format, $tzID );
}

I think we need a separate function that reliably gets the correct timezone string. This function should be called in the place of lines 41 - 47 in your code above if the user doesn't explicitly set the timezone.

This is what I have come up with:

function get_timezone() {
    $timezone = get_option( 'timezone_string' );
    if( empty( $timezone ) ) {
        $timezone = sprintf( 'UTC%+.4g', get_option( 'gmt_offset', 0 ) );
    }
    return $timezone;
}

Lines 48 - 50 can be condensed to this:

$date = DateTime::createFromFormat( 'U', strtotime( $date_string ), new DateTimeZone( $tzID ) );

NOTE: In actual testing, this didn't seem to properly set the timezone.

So rewriting the iso8601_gmt_to_local() function would look like this:

function iso8601_gmt_to_local( $date_string, $format = 'c', $timezone = null ) {
    $timezone = is_null( $timezone ) ? get_timezone() : $timezone;
    $date = DateTime::createFromFormat( 'U', strtotime( $date_string ) );
    $date->setTimezone( new DateTimeZone( $timezone ) );
    return $date->format( $format );
}

https://gist.github.com/dfwood90/6196705#file-wp-iso8601-datetime-php-L46

I believe this should be 'Etc/GMT' . ( ( $offset < 0 ) ? $offset : '+' . $offset ); should it not?