Formatting a ToString for time

Hi, I’m trying to make a digital clock in C# and I am having trouble with the formatting. I am trying to get a countdown from 5 min and display it in a mm:ss format. Here is what I have so far, but I can’t figure out how to adjust the formatting in the ToString.

currentTime = nextDay - Time.time;
min = Mathf.Ceil(currentTime/60);
sec = Mathf.Ceil (currentTime % 60);
var timer = min + ":" + sec;

GUI.Label (new Rect(712,495,50,30), timer.ToString());

You can use the C# String.Format Method.
This could look something like this (for mm:ss format):

    public static string SecondsToString(int sec)
        int minutes = sec / 60;
        int seconds = sec % 60;

        return string.Format("{0:00}:{1:00}", minutes, seconds);

min = Mathf.Ceil(currentTime/60);
min_s = min.ToString();

while (min_s.Length < 2)
 min_s = "0" + min_s;

Didn’t test it.