Calculate flight time - timezone

Question

I am working now on a function ot calculate the flight time. I have a very strange problem, because it is a new domain for me.
Here is one example:
Outward:
25.02.2011 11:45 Cologne - Las Vegas 26.02.2011 21:18
Return:
18.03.2011 09:49 Las Vegas - Cologne 19.03.2011 11:05
For this flight Kayak is showing
Outward: 18:33 hours
Return: 17:16 hours
Cologne is GMT +01:00
Las Vegas is America/Los_Angeles timezone -> -08:00
But in the example the flight durariion for the outward flight is calculated with -8 hours offset for Las Vegas and for the return flight with -7 hours offset for Las Vegas. Why?
I have see the timezones here: http://download.geonames.org/export/dump/timeZones.txt
Has anyone any idea?
Thanks
Nik

Answer 1

It's because Daylight Savings Time doesn't start on the same date everywhere.
Las Vegas: DST starts on Sunday, 13 March 2011
Cologne: DST starts on Sunday, 27 March 2011
So since your return flight is on the 18th of March, Las Vegas will be in DST, but Cologne won't be.

Answer 2

Ok, I solve the problem. Thanks for the advice to convert both dates to UTC/GMT first.
Here is the trick:
$reset_timezone = date_default_timezone_get();
// convert the first time
date_default_timezone_set($timezone_identifier1);
$time1 = $date_from.' '.$time_from;
$gmtime1 = gmdate('Y-m-d H:i', strtotime($time1));
// convert the second time
date_default_timezone_set($timezone_identifier2);
$time2 = $date_to.' '.$time_to;
$gmtime2 = gmdate('Y-m-d H:i', strtotime($time2));
// reset the timezone again
date_default_timezone_set($reset_timezone);
--> here calculate the difference < --
Hope it helps anyone else!
Cheers
Nik