How to format 1800 seconds to MM:SS format

You get the numbers by integer division and modulo (%)
and the formatting to 2 digits with padStart():

val secs = 1800
val formatted = "${(secs / 60).toString().padStart(2, '0')} min : ${(secs % 60).toString().padStart(2, '0')} sec"
println(formatted)

will print

30 min : 00 sec

Since you are targeting the JVM, you should use the Java 8 Date/Time Api. You could create a very concise function like this:

fun countdown(s: Long) = with(LocalTime.ofSecondOfDay(s)) {
    String.format("%02d min :%02d sec", minute, second)
}

countdown(1800) // 30 min : 00 sec

Recommendation:

• Don't do the calculations at call site (in onTick). This makes the code unecessary hard to understand. Create a separate function for that.

• It's a simple calculation, but use what the standard libraries give you. Firstly, the code is optimized and secondly you can easily extend the function to calculate hours and so on. Untested, hand-drafted code is error prone.

Here's how my code looks like at the moment:

val timer = object: CountDownTimer(1800000, 1000) {

    override fun onTick(millisUntilFinished: Long) {

        val timeResult = 
            "${(millisUntilFinished / 1000 / 60).toString().padStart(2, '0')}:" +
            "${(millisUntilFinished / 1000 % 60).toString().padStart(2, '0')} "

        textTimer.text = "$timeResult"
    }
    override fun onFinish() {
        textTimer.text = "Time is out"
    }
}
timer.start()
textTimer.text = "$timer"

Following code can convert from milliseconds (which is what you have initially) to minutes and seconds:

val milliseconds: Long = 1800000

val minutes = milliseconds / 1000 / 60
val seconds = milliseconds / 1000 % 60

println("$minutes min : $seconds sec")

Output:

30 min : 0 sec

EDIT 1:

If you strictly need it from seconds to minutes and seconds, just remove the extra division by 1000.

The code for that will be:

val seconds: Long = 1800

val minutes = milliseconds / 60
val seconds = milliseconds % 60

println("$minutes min : $seconds sec")

The code in the original question had milliseconds in it that's why I mentioned it from Milliseconds.

EDIT 2:

Missed out the double digit formatting.

The answer by @forpas using .toString().padStart(2, '0') will give the correct formatted output. So, the correct code would be:

val milliseconds: Long = 1800000

val minutes = milliseconds / 1000 / 60
val seconds = milliseconds / 1000 % 60

println("{$minutes.toString().padStart(2, '0')} min : {$seconds.toString().padStart(2, '0')} sec")

Output:

30 min : 00 sec