PHP Unix Seconds time to PHP UTC time with Milliseconds

Question

is there a way to convert a Unix Seconds time in PHP to a PHP UTC time with Milliseconds? I'm trying to pass the time to the datepicer property in HubSpot but I can't get the API to accept what it wants.

Here's what I got:

$time = 1632182399;

I had did some research and found a contributor had suggested this solution: https://community.hubspot.com/t5/APIs-Integrations/Update-Custom-Date-Picker-in-Deals-through-the-API/m-p/229499

But this doesn't actually work and I don't know why. Perhaps I'm calling it wrong? This is what I was trying:

function get_time_for_hubspot($time){
    $target_date = date( 'Y-m-d', strtotime( $time ) );
    $timezone_object = new \DateTimeZone( 'UTC' );
    $date_object = new \DateTime( $target_date, $timezone_object );
    return $date_object->format( 'U' ) * 1000;
}

 // Time Stamps
    $starttime = strval($booking_object->start);

    $endtime = strval($booking_object->end);

    $starttime = get_time_for_hubspot($starttime);
    $endtime = get_time_for_hubspot($endtime);

Thanks for any help.