We can use strtotime() function to achieve this. For example:
$date=date('Y-m-d',time());
$datestamp=strtotime(date('Y-m-d',strtotime($date)).' +1 day');
$datestamp=strtotime(date('Y-m-d',strtotime($date)).' +1 week');
$datestamp=strtotime(date('Y-m-d',strtotime($date)).' +1 month');
Here strtotime() will return a timestamp if success.
For subtraction,we can have:
$date=date('Y-m-d',time());
$datestamp=strtotime('-1 day',$strtotime($date));
$datestamp=strtotime('-1 week',$strtotime($date));
$datestamp=strtotime('-1 month',$strtotime($date));
We can use this when we want to set a deadline or remind date in our web application.