How do I check if a timestamp is in the DST daylight-saving hour?

Question

The problem is, that for example 2:00am or 2:59am are doubled in autumn in some timezones and don't exist in spring during the DST time-saving change.

If I run a PHP loop through every minute through a whole year, how can I catch the DST timesavings hour within the loop? (in the current Timezone, set by date_default_timezone_set)

How would I complete a PHP 5.2 compatible function like:

<?php
/** returns true if a time is skipped or doubled 
 * (for example "2013-03-10 02:30" doesen't exist in USA)
 * 
 * @param string $season
 * @param string $datestring in the form of "2013-03-10 02:30"
 * @return boolean
 **/
function checkDST($datestring,$season="any"){
    $tz=date_default_timezone_get();
    $season=strtolower(trim($season));
    if($season=="spring"){
        if(/* an hour skipped */) return true;  
    }else if($season=="autumn"){
        if(/* double hour */) return true;  
    } else if($season=="any") {
        if(/* any of both */) return true;
    }
    return false
}

so I could use

date_default_timezone_set("America/New_York");
$is_skipped=checkDST("2013-03-10 02:30","spring");  

and

$exists_two_times=checkDST(2003-11-03, 02:00,"autumn"); // or 2:59 for example

(Timezones see: http://www.timeanddate.com/worldclock/clockchange.html?n=224&year=2013 )

---

EDIT:
I found out how to detect the spring DST:

function checkDST($datestring,$season="any"){
    var_dump('checking '.$datestring);
    $season=strtolower(trim($season));
    $datestring=substr($datestring,0,16);
    if($season!="autumn" and date("Y-m-d H:i",strtotime($datestring))!=$datestring) {
        return true;
    }
    // check for double hours in autumn
    ...

Answer 1

This works on all times in

date_default_timezone_set("America/New_York");

There is only still one bug, if I call it for europe, it sais the hour is one hour early:

date_default_timezone_set("Europe/Berlin");
var_dump(checkDST("2013-10-27 01:30","any"));
var_dump(checkDST("2013-10-27 02:30","any"));

(The savings hour in europe is from 2am till 3am)

This is the conclusion so far:

<?php
/** returns true if a time is skipped or doubled 
 * (for example "2013-03-10 02:30" doesen't exist in USA)
 * 
 * @param string $season "spring", "autumn", "fall" or any other string for any of the time changes
 * @param string $datestring in the form of "2013-03-10 02:30"
 * @return boolean
 **/
function checkDST($datestring, $season, $tz = null){
    $debug=true;
    if($debug) echo ('checking '.$season.' '.$datestring);
    $season=strtolower(trim($season));
    if($season=="fall") $season="autumn";
    $datestring=substr($datestring,0,16);
    $monthdaystring=substr($datestring,0,10);
    if(!strtotime($datestring) or date("Y-m-d",strtotime($monthdaystring))!=$monthdaystring) {
        //Error
        if($debug) echo "<br>Error: not a correct datestring: $datestring";
        return false;
    }
    if($season!="autumn" and date("Y-m-d H:i",strtotime($datestring))!=$datestring) {
        // spring or any
        if($debug) echo "<br>".(date("Y-m-d H:i",strtotime($datestring)).'!='.$datestring);
        return true;
    } else if($season=="spring") {
        // spring ends here
        return false;
    }
    // "autumn" or "any" 
    $timestamp = strtotime($datestring);

    $timestamp_start = new DateTime();
    $timestamp_start->setTimestamp($timestamp);
    $timestamp_end = new DateTime();
    $timestamp_end->setTimestamp($timestamp)->add(new DateInterval('PT3600S'));

    if (!$tz) $tz = date_default_timezone_get();
    $timezone = new DateTimeZone($tz);
    $transitions = $timezone->getTransitions($timestamp_start->getTimestamp(), $timestamp_end->getTimestamp());
    if (count($transitions) > 1) { // there's an imminent DST transition, spring or fall
        // autumn
        if($debug) echo "<br>NumTransitions: ".(count($transitions));
        if($debug) var_dump($transitions);
        return true;
    }
    return false;
}

Answer 2

If you're using PHP's date_default_timezone_set to parse what I assume to be Unix timestamps, the timestamps will be parsed relative to that timezone. To then determine the Daylight Savings Time offset, if one is relevant, use DateTimeZone::getTransitions.

A function that worked similar to the one in your example might look like this:

<?php
function checkDST($datestring, $season, $tz = "America/New_York", $minutes_from_now_to_check = 1){
    $seconds_to_check = ($minutes_from_now_to_check * 60) + 30;
    if (!$tz) $tz = date_default_timezone_get();
    $timestamp = strtotime($datestring);
    $timestamp_start = new DateTime();
    $timestamp_start->setTimestamp($timestamp);
    $timestamp_end = new DateTime();
    $timestamp_end->setTimestamp($timestamp)->add(new DateInterval('PT'.$seconds_to_check.'S'));

    $timezone = new DateTimeZone($tz);
    $transitions = $timezone->getTransitions($timestamp_start->getTimestamp(), $timestamp_end->getTimestamp());
    if (count($transitions) > 1) { // there's an imminent DST transition, spring or fall
        if (strtolower($season) == "spring" && $transitions[1]["isdst"] === true){
            return true;    
        } 
        if (strtolower($season)=="autumn" && $transitions[1]["isdst"] === false){
            return true;
        }
        if (strtolower($season)=="any"){
            return true;
        }
    }
    return false;
}

$is_skipped = checkDST("2013-03-10 01:59","spring");
var_dump($is_skipped); // will display bool(true)

$is_skipped = checkDST("2013-03-10 12:30","spring");
var_dump($is_skipped); // will display bool(false)

$exists_two_times=checkDST("2013-11-03 01:59","autumn");
var_dump($exists_two_times); // bool(true)

$exists_two_times=checkDST("2013-11-03 03:00","autumn");
var_dump($exists_two_times); // bool(false)