The time() function call should always be a larger number than th value used in the strtotime() call. So it should go first.
Also that value is in seconds and its the difference, so you would need to divide by the number of seconds in a hour (3600) to get the number of hours, the number of seconds in a day (86400) to get the number of days and so on. Using any of the date functions on that differnece value will not work