Time to final dual?

Now, my brain is messed up. How do I count the time to dual?
My timers code:

PHP:

            $timetod = $this->totalminutes - $this->minutespast + $this->prefs->get('dual_time');

This is a scheduleRepeatingTask with 1200 ticks(1 minute).
This code will count the numbers up. How do I make it count down the correct way?

What do you mean..

I recommend you to use time() or microtime(true), because they give you the real time instead of the server ticks which may be affected by TPS.
Don't count the minutes yourself. Compare time() and saved timestamps directly. time() gives the number of seconds (in integer) passed since 1/1/1970 UTC 0:00:00.

PHP:

initialize:
$this->startTime = time(); // saves the time for timer start

seconds_after_initialize:
$secondsElapsed = time() - $this->startTime; // gets the number of seconds pased since timer start

// if the timer should end 5 minutes after initialize
timestamp_for_end:
$endTime = $this->startTime + 60 * 5; // adds 300 seconds (5 minutes) to the timer start time, which is the timer end time
seconds_left:
$secondsLeft = $endTime - time(); // difference between timer end time and current time is the number of seconds left.