Time calculation in php (add 10 hours)?

In order to increase or decrease time using strtotime you could use a Relative format in the first argument.

In your case to increase the current time by 10 hours:

$date = date('h:i:s A', strtotime('+10 hours'));

In case you need to apply the change to another timestamp, the second argument can be specified.

Note:

Using this function for mathematical operations is not advisable. It is better to use DateTime::add() and DateTime::sub() in PHP 5.3 and later, or DateTime::modify() in PHP 5.2.

So, the recommended way since PHP 5.3:

$dt = new DateTime(); // assuming we need to add to the current time
$dt->add(new DateInterval('PT10H'));
$date = $dt->format('h:i:s A');

or using aliases:

$dt = date_create(); // assuming we need to add to the current time
date_add($dt, date_interval_create_from_date_string('10 hours')); 
$date = date_format($dt, 'h:i:s A');

In all cases the default time zone will be used unless a time zone is specified.