Subtracting time from variable in PHP

I have a variable last_start which is part of an array in $u. I would like to subtract 20 seconds from last_start variable in my PHP code.

private function _process_url(Employ $e, url $u, $job_order, array $urls_processed) 
    $summary = [
        'url_id' => $u->get_id(),
        'start_time' => date('Y-m-d H:i:s'),
        'success_count' => 0,
        'error_count' => 0
    ];
    $u->last_start = new DateTime();
    $u->update(['last_start']);

I have a question on this code part:

$u->last_start = new DateTime();
$u->update(['last_start']};

I have tried subtracting 20 seconds from last_start variable nut apparently it is incorrect this way. It does not work. as it gives an error:

Uncaught exception: Call to undefined function modify()

How to fix this?

$u->last_start = new DateTime();
$u->last_start = modify("-20 second");
$u->update(['last_start']};

Solution:

All that was needed was:

$u->last_start = new DateTime();
$u->last_start = new DateTime('-20 sec');
$u->update(['last_start']);