Print the time an hour ago

$date = date('Y-m-d H:i:s', strtotime('-1 hour'));
echo 'John visited last ' . $date;

$date = date("Y-m-d H:i:s", time() - 3600);

time() -> Current timestamp

Time minus 3600 seconds, is the time 1 hour ago. To get the date formatted, you can look here for the options: http://php.net/manual/en/function.date.php

Alternatively you could use the following format:

$date = date("Y-m-d H:i:s", strtotime('-1 hour'));

Though using that method can be a little clunky if you want to remove more specific units of time (Such as one day and 3 hours).

Thats if I've understood what you want to do correctly that is.