Google Analytics PHP: get all events with specific label since the beginning

I know I can get all events with a specific label in an interval, but I was wondering how can I get all events with that label since the first event occurred. Something to replace $startDate with some magic value that means "beginning"

$data = $analytics->data_ga->get(
    'ga:xxxxxxxxx',
    $startDate, // !!!
    'today',
    'ga:totalEvents',
    [
        'dimensions' => 'ga:eventLabel,ga:date',
        'filters' => "ga:eventLabel=={$this->type}"
    ]
);