A script to guess the timezone based on data provided in a VTIMEZONE object in an iCal file. The name is a non-olson-name. Those with olson-names are easy. Originally written for https://github.com/fruux/sabre-vobject/issues/44

guessTimeZone.php

<?php

$timezones = DateTimeZone::listIdentifiers();

$VTIMEZONE = array(
    "TZID" => "Mitteleuropäische Zeit",
    "DAYLIGHT" => array(
        "RULE" => array(
            "FREQ" => "YEARLY",
            "BYDAY" => "-1SU",
            "BYMONTH" => 3,
        ),
        "DTSTART" => "20140330T010000",
        "TZOFFSETFROM" => "+0100",
        "TZOFFSETTO" => "+0200",
    ),
    "STANDARD" => array(
        "RULE" => array(
            "FREQ" => "YEARLY",
            "BYDAY" => "-1SU",
            "BYMONTH" => 10,
        ),
        "DTSTART" => "19701101T020000",
        "TZOFFSETFROM" => "+0200",
        "TZOFFSETTO" => "+0100",
    )
);

function parseOffsetToInteger($offset) {

    $time = ($offset[1].$offset[2] * 60) + ($offset[3].$offset[4]);
    
    // in seconds please ..
    $time = $time * 60;

    if ($offset[0] === "-") {
        $time = $time *-1;
    }
    
    return $time;
}

// Source: http://stackoverflow.com/questions/924246/get-the-first-or-last-friday-in-a-month
function get_date($month, $year, $week, $day, $direction) {
    if($direction > 0)
        $startday = 1;
    else
        $startday = date('t', mktime(0, 0, 0, $month, 1, $year));

    $start = mktime(0, 0, 0, $month, $startday, $year);
    $weekday = date('N', $start);

    if($direction * $day >= $direction * $weekday)
        $offset = -$direction * 7;
    else
        $offset = 0;

    $offset += $direction * ($week * 7) + ($day - $weekday);
    return mktime(0, 0, 0, $month, $startday + $offset, $year);
}

foreach($timezones as $timezone) {
    if ($timezone !== "Europe/Berlin")
        continue;
    
    $timeZones = array();
    $timeZones[] = parseOffsetToInteger($VTIMEZONE["DAYLIGHT"]["TZOFFSETFROM"]);
    $timeZones[] = parseOffsetToInteger($VTIMEZONE["DAYLIGHT"]["TZOFFSETTO"]);

    $timeZones[] = parseOffsetToInteger($VTIMEZONE["STANDARD"]["TZOFFSETFROM"]);
    $timeZones[] = parseOffsetToInteger($VTIMEZONE["STANDARD"]["TZOFFSETTO"]);
    
    // Reduce the list of timezones to those, who're valuable to take a closer look at.
    $now = new DateTime("now", new DateTimeZone($timezone));
    if (!in_array($now->getOffset(), $timeZones)) {
        continue;
    } else {
        echo "Matched one of the timezones: $timezone\n";
    }

    // Guess timezone based on DTSTART
    $transitions = timezone_transitions_get(new DateTimeZone($timezone));
    foreach($transitions as $transition) {
        foreach(array("DAYLIGHT" => $VTIMEZONE["DAYLIGHT"], "STANDARD" => $VTIMEZONE["STANDARD"]) as $name => $definition) {
            try {
                if ((new DateTime($definition["DTSTART"]))->format("c") == (new DateTime($transition["time"]))->format("c")
                   && $transition["offset"] === parseOffsetToInteger($definition["TZOFFSETTO"])) {
                    echo "[$name] Timezone found by DTSTART: $timezone\n";
                }
            } catch (Exception $e) {
                if (strpos($e->getMessage(), "DateTime::__construct(): Failed to parse time string") !== 0) {
                    throw $e;
                }
            }
        }
    }

    // Guess timezone based on RULES for now
    foreach(array("DAYLIGHT" => $VTIMEZONE["DAYLIGHT"], "STANDARD" => $VTIMEZONE["STANDARD"]) as $name => $definition) {
        $rules = $definition["RULE"];

        if ($rules["FREQ"] !== "YEARLY") {
            echo "- [$name] Unknown frequency {$rules['FREQ']}";
            continue;
        }

        $dayMap = array(
            "MO" => 1,
            "TU" => 2,
            "WE" => 3,
            "TH" => 4,
            "FR" => 5,
            "SA" => 6,
            "SU" => 7,
        );

        date_default_timezone_set('UTC');
        $date = date(DATE_ATOM, get_date(
            $rules["BYMONTH"],
            date("Y"),
            $rules["BYDAY"][1], // Count of weeks to go back or forth
            $dayMap[$rules["BYDAY"][2].$rules["BYDAY"][3]], // Weekday as number
            $rules["BYDAY"][0] === "-" ? -1 : 1
        ));
        
        $dtstart = new DateTime($definition["DTSTART"]);
        
        $dateTime = new DateTime($date);
        $dateTime->setTime(
            $dtstart->format("H")+0,
            $dtstart->format("i")+0,
            $dtstart->format("s")+0
        );

        // Set time back for 1sec to get behind the time-transition
        $dateTime->sub(new DateInterval("PT1S"));

        // Try the first offset
        $dateTime->setTimezone(new DateTimeZone($timezone));

        if ($dateTime->getOffset() !== parseOffsetToInteger($definition["TZOFFSETFROM"])) {
            echo "- [$name] Warning for TZOFFSETFROM: {$dateTime->getOffset()} - {$definition['TZOFFSETFROM']} | ({$timezone})\n";

            // Maybe the system set one hour too late for the switch to StandardTime? Like M$ does in the provided example ... :)
            if (parseOffsetToInteger($definition["TZOFFSETFROM"]) < parseOffsetToInteger($definition["TZOFFSETTO"])) {
                // Set it back to UTC, add two seconds to get past the time-transition
                $dateTime->setTimezone(new DateTimeZone("UTC"));
                $dateTime->sub(new DateInterval("PT1H"));

                // Try again for the next offset
                $dateTime->setTimezone(new DateTimeZone($timezone));

                if ($dateTime->getOffset() !== parseOffsetToInteger($definition["TZOFFSETTO"])) {
                    echo "- [$name] Skipped for TZOFFSETTO: {$dateTime->getOffset()} - {$definition['TZOFFSETTO']} | ({$timezone})\n";
                    continue 2;
                }
            }
        }

        // Set it back to UTC, add two seconds to get past the time-transition
        $dateTime->setTimezone(new DateTimeZone("UTC"));
        $dateTime->add(new DateInterval("PT2S"));
        
        // Try again for the next offset
        $dateTime->setTimezone(new DateTimeZone($timezone));
        
        if ($dateTime->getOffset() !== parseOffsetToInteger($definition["TZOFFSETTO"])) {
            echo "- [$name] Skipped for TZOFFSETTO: {$dateTime->getOffset()} - {$definition['TZOFFSETTO']} | ({$timezone})\n";
            
            continue 2;
        }

        echo "[$name] Timezone found by RULE: $timezone\n";
    }

    // Guess timezone based on RULES for registered timezone-transitions
    // TODO: Add another loop, that checks if the timezone-transitions match the defined timezone-rules
}