Date and time in php: common function

Convert a date in a timestamp:

strtotime ( string $datetime [, int $now = time() ] ) : int

https://www.php.net/manual/en/function.strtotime.php

Add an arbitrary amount of time in a string format:

strtotime("+1 week"), "\n";

$availability_date=get_post_meta( get_the_ID(), 'availability date', true );
$date_string=strval($availability_date);
$date_string.=" +3 weeks";
$date=new DateTime;
$diplay_time= date('d-m-Y',strtotime($date_string)) ;
$availability_date=strtotime($availability_date);