I have a invoicing system that generates the next due date after the invoice is paid. My problem though is I want to generate the next invoice based on the date the last one was due, not when it was paid.
I'm familiar with adding days to the current date such as this:
$nextduedate = date('Y-m-d', strtotime("+30 days"));
$nextduedate = ('Y-m-d', strtotime($duedate. ' + 30 days'));
That will format your date, then add 30 days to the old due date stored in a variable.