Difference between two times in Android Studio, (Registered time and current time)

I have a simple code, I record the time when the user hits a button and subtract that time from the current time. It should give me the difference.

The funny part is that if I print my vars they are correct but when I make the subtraction the result is the year 1969 the time the system takes as reference.

This is my code:

// I register this time to later compare to the current time.
startTime = System.currentTimeMillis().toDouble()

timer = Timer()
timer.schedule(timerTask {
    runOnUiThread {
        advanceTimer()
    }
}, 0, 60)

fun advanceTimer() {
//Total time since timer started, in seconds
val currentTime = System.currentTimeMillis().toDouble()
time = currentTime-startTime

}

This is what the console prints:

D/startTime: 2020:12:20:21:27:39:78

D/currentTime: 2020:12:20:21:29:49:21

D/time: 1969:12:31:18:02:09:42

Can somebody help me, please?

Solution

This answer lead me the right way, actually my approach was good I just needed to make a function that showed only the difference.

I made this one. Hope it helps somebody.

Just call the function with the result of you operation, at the end I used Date().getTime() instead of System.currentTimeMillis() but I think it should be the same.

The complete code:

startTime = System.currentTimeMillis().toDouble()

timer = Timer()
timer.schedule(timerTask {
    runOnUiThread {
        advanceTimer()
    }
}, 0, 60)

fun advanceTimer() {
//Total time since timer started, in seconds
val currentTime = System.currentTimeMillis().toDouble()
time = currentTime-startTime

// Shows the time in a label on the screen
timerString.text = differenceResult(time)
}

fun differenceResult(time: Long): String {
    var x: Long = time / 1000

    var seconds = x % 60
    x /= 60
    var minutes = x % 60
    x /= 60
    var hours = (x % 24).toInt()
    x /= 24
    var days = x
    return String.format("%02d:%02d:%02d", hours, minutes, seconds)
}