Adding Days to Date
No worries, try this (added concatenation operator "." before ' + 15 days:
echo date('Y-m-d', strtotime($invoicesRecord['createdDate'] .' + 15 days'));
Also, sometimes what I do what the there's a lot of syntactic noise (operators and characters all bunched together) is break it down into multiple simple statements:
$createdDate = $invoicesRecord['createdDate'];
$dateIn15Days = strtotime("$createdDate + 15 days");
echo date('Y-m-d', $dateIn15Days);
Hope that helps!