The following is really easy way to add days, minutes, hours and seconds to a time using PHP. Using the date function to set the format of the date to be returned then using strtotime to add the increase or decrease of time then after a comma use another strtotime passing in the start date and time.

//set timezone
date_default_timezone_set('GMT');

//set an date and time to work with
$start = '2014-06-01 14:00:00';

//display the converted time
echo date('Y-m-d H:i',strtotime('+1 hour +20 minutes',strtotime($start)));

Times can be entered in a readable way:

• +1 day = adds 1 day
• +1 hour = adds 1 hour
• +10 minutes = adds 10 minutes
• +10 seconds = adds 10 seconds

To sub-tract time its the same except a - is used instead of a +