how to use mktime with "I" (summertime option)?

I wanted to code a script which returns true if the given date is in the summertime. So I found date("stuff" mktime) in the php documentation. And there is a list of parameter strings. There it says: "I (capital i) Whether or not the date is in daylight saving time". Some lines below I found this example:

echo "July 1, 2000 is on a " . date("l", mktime(0, 0, 0, 7, 1, 2000));

That's why I wrote this in my script:

function summertime(string $date) : bool {
    $pieces = explode('-', $date);
    $year = (int) $pieces[0];
    $month = (int) $pieces[1];
    $day = (int) $pieces[2];

    return date("I", mktime(0, 0, 0, $month, $day, $year));
}

echo summertime("1981-07-07");

No matter which date I put in, it returns always false/0 what ever... I can't find the difference or mistake... FYI: I am using PHP 7.0.

As noted in Markus Laire's comment, you probably need to set your timezone correctly. I'd also recommend using the DateTime class. The examples below show how to set a specific timezone when creating a new date:

Example 1:

$tz = new DateTimeZone('Europe/Berlin');
$date = new DateTime('1981-07-07', $tz);
echo $date->format('I');
// outputs '1'

Example 2:

$tz = new DateTimeZone('Europe/Berlin');
$date = new DateTime('1981-01-01', $tz);
echo $date->format('I');
// outputs '0'