As reported through the [[http://lists.kolab.org/pipermail/devel/2015-June/015337.html | devel mailing list], `libvacalendaring` generates invalid timezone specifications specifically with TZOFFSETFROM and TZOFFSETTO for timezones with negative UTC offsets:
```
BEGIN:VTIMEZONE
TZID:America/New_York
X-MICROSOFT-CDO-TZID:10
BEGIN:STANDARD
DTSTART:20141102T060000
TZOFFSETFROM:-400
TZOFFSETTO:-500
TZNAME:EST
END:STANDARD
...
END:VTIMEZONE
```
The offset values should read `-0400` and `-0500` instead.
The reporter suggests this {F10596} for fixing the issue.
**Test cases to be created:**
```
$tz = new DateTimezone('America/New_York');
$event = array(
'uid' => '12345',
'summary' => 'test',
'start' => new DateTime('20130703T143000', $tz),
'end' => new DateTime('20130703T143000', $tz),
);
$ical = new libvcalendar();
$ics = $ical->export(array($event));
```
**Expected result:**
`$ics` contains `TZOFFSETFROM:-0400` and `TZOFFSETTO:-0500`