Fixing Validation Errors

While testing importing the iCal into non-Apple calendars I consistently get errors. When I run the iCal feed through a validator it throws errors like this:
Error at line 11:Cannot set timezone for UTC

It points to DSTAMP and the way the plugin uses timezones. Then I found a bunch of articles pointing to the need for only using the Z format.

So I’m suggesting these changes:

change this

$start_time = date( 'Ymd\THis', get_post_time( 'U', false, $post->ID ) );
$end_time = date( 'Ymd\THis', get_post_time( 'U', false, $post->ID ) + ($options['icalfeeds_minutes'] * 60));

to this:

$start_time = date( 'Ymd\THis', get_post_time( 'U', true, $post->ID ) );
$end_time = date( 'Ymd\THis', get_post_time( 'U', true, $post->ID ) + ($options['icalfeeds_minutes'] * 60));

And then change this:

$modified_time = date( 'Ymd\THis', get_post_modified_time( 'U', false, $post->ID ) );

to this:

$modified_time = date( 'Ymd\THis', get_post_modified_time( 'U', true, $post->ID ) );

and finally change this:

if ($timezone === 'UTC') {
            $start_time = ":$start_time" . 'Z';
            $end_time = ":$end_time" . 'Z';
            $modified_time = ":$modified_time" . 'Z';
} else {
            $start_time = ";TZID=$timezone:$start_time";
            $end_time = ";TZID=$timezone:$end_time";
            $modified_time = ";TZID=$timezone:$modified_time";
}

to this:

$start_time = ":$start_time" . 'Z';
$end_time = ":$end_time" . 'Z';
$modified_time = ":$modified_time" . 'Z';

Hope that helps.