PHP DateTime converting issue - php

Question

I have an issue with converting DateTime from Moscow timezone to New York timezone. Here is my test script:
$year = 2015;
$month = 3;
$tzMoscow = new DateTimeZone('Europe/Moscow');
$tzNewYork = new DateTimeZone('America/New_York');
$startDate = DateTime::createFromFormat('Y-n-d', "$year-$month-01", $tzMoscow);
echo $startDate->format('Y-m-d H:i:s')."\n"; // 2015-03-01 16:16:05
$startDate = DateTime::createFromFormat('Y-n-d', "$year-$month-01", $tzNewYork);
echo $startDate->format('Y-m-d H:i:s') . "\n"; // 2015-03-01 09:16:05
$startDate->setTimezone($tzMoscow);
echo $startDate->format('Y-m-d H:i:s') . "\n"; // 2015-03-01 17:16:05
Third output is incorrect, time should be 16:16:05. Am I doing something wrong or is this a bug in php?

Answer 1

I think I've got it. The problem is that you're not specifying a time of day, so createFromFormat is using "the current system time":
If format does not contain the character ! then portions of the generated time which are not specified in format will be set to the current system time.
Now what does it mean to use "the current system time" to create a time in the New York time zone, when that time zone has changed UTC offset between the specified date (March 1st) and the current date (March 14th)? It was UTC-5 on March 1st, and it's now UTC-4 due to daylight saving time.
I believe PHP is taking the current time of day in the specified time zone (9:16 at the time you ran that code) and then using that as the time of day on the specified date. So we end up with March 1st 2015, 09:16 New York time - or March 1st, 14:16 UTC, which is indeed March 1st 17:16 Moscow time.
That's not the same as the current time of day in Moscow at the time that you ran the code, which was 16:16.
Basically, you should try not to do this - or expect problems like this to happen. Think about what time of day you're really trying to represent, bearing in mind that within a particular time zone, offsets change over time. I can't really advise you on what your code should be, because we don't know what you're trying to achieve - but using the current time of day for a different date can definitely cause this problem.

Answer 2

I also believe this is an issue of DST, changing in New York time zone on 8th of March. I extended your date formats to Y-m-d H:i:s U I to include the unix timestamp and the DST value.
The output is now as following:
2015-03-01 19:07:17 1425222437 0
2015-03-01 11:07:17 1425226037 0
2015-03-01 20:07:17 1425226037 0
As you can see, the unix timestamp is already different between both created DateTime objects.
Now when I specify a concrete time as
$startDate = DateTime::createFromFormat('Y-n-d H:i', "$year-$month-01 00:00", $tzMoscow);
as well as
$startDate = DateTime::createFromFormat('Y-n-d H:i', "$year-$month-01 00:00", $tzNewYork);
the output changes to:
2015-03-01 00:00:00 1425153600 0
2015-03-01 00:00:00 1425186000 0
2015-03-01 09:00:00 1425186000 0
On the other hand, if I change $month to 4 (where New York uses DST), I get the following output:
2015-04-01 19:08:35 1427900915 0
2015-04-01 11:08:35 1427900915 1
2015-04-01 19:08:35 1427900915 0
What do these results mean?
The conversion between New York and Moscow time zone works correct in all cases, as you can judge from the unix timestamp being identical. Also, "2015-03-01 00:00" are obviously different timestamps for Moscow and New York, because they depend on the concrete time zone.
So I think your code is correct and there is no bug in php. However, the "current" times in New York and Moscow differ due to the DST switch between 1st of March and today (14th of March).
So while Jon's answer already explained the theory (I don't want all the credit, he was first), maybe someone will still find some concrete examples useful.

Answer 3

You can use this function, for change between timezones
function changeTimezone($time, $currentTimezone, $timezoneRequired, $FormtsTime = 'Y-m-d h:i:s')
{
$dayLightFlag = false;
$dayLgtSecCurrent = $dayLgtSecReq = 0;
$system_timezone = date_default_timezone_get();
$local_timezone = $currentTimezone;
date_default_timezone_set($local_timezone);
$local = date($FormtsTime);
date_default_timezone_set("GMT");
$gmt = date($FormtsTime);
$require_timezone = $timezoneRequired;
date_default_timezone_set($require_timezone);
$required = date($FormtsTime);
date_default_timezone_set($system_timezone);
$diff1 = (strtotime($gmt) - strtotime($local));
$diff2 = (strtotime($required) - strtotime($gmt));
$date = new DateTime($time);
$date->modify("+$diff1 seconds");
$date->modify("+$diff2 seconds");
if ($dayLightFlag) {
$final_diff = $dayLgtSecCurrent + $dayLgtSecReq;
$date->modify("$final_diff seconds");
}
$timestamp = $date->format($FormtsTime);
return $timestamp;
}