(This is only here for personal reference, since this is rather basic programming …)
minutes = Mathf.FloorToInt(input / 60);
seconds = Mathf.FloorToInt(input - minutes * 60);
string r = (minutes < 10) ? "0" + minutes.ToString() : minutes.ToString();
r += ":";
r += (seconds < 10) ? "0" + seconds.ToString() : seconds.ToString();
return r;
}