setting accountexpires date with PHP and LDAP

Question

I'm having trouble setting the 'accountexpires' and wondered if anyone knows a way around it, I believe it's because I have a 18 digit number. I have a form where someone sets a date, I need to set the account to expire then, if I provide the number as a hardcoded string it works, but I cannot convert the finished calculation to a string. Anyone know any way around this? thanks.

<?php
$currentTimeUnix = 1442383989; //this will vary depending on user's input
$secondsBetween1601and1970 = 11644473600;
$timesAdded = $currentTimeUnix + $secondsBetween1601and1970;// comes to 13086857589
$nanoseconds = $timesAdded * 10000000; //comes to 130868575890000000
echo $nanoseconds; //displays 1.3086857589E+017
echo (string)$nanoseconds; //displays 1.3086857589E+017
echo strval($nanoseconds); //displays 1.3086857589E+017
?>

Answer 1

Thanks for this. Been testing true out the net but nothing worked until i stumbled on this solution:

$expiredate = strtotime($values["expires"]);
$currentTimeUnix = $expiredate;
$secondsBetween1601and1970 = 11644473600;
$timesAdded = $currentTimeUnix + $secondsBetween1601and1970;
$nanoseconds = $timesAdded * 10000000;
$expires = sprintf('%018.0f',$nanoseconds);