Here's the bit of code I'm using to display floating point seconds as MM:SS.FF ...
string FormatSeconds(float elapsed)
{
int d = (int)(elapsed * 100.0f);
int minutes = d / (60 * 100);
int seconds = (d % (60 * 100)) / 100;
int hundredths = d % 100;
return String.Format("{0:00}:{1:00}.{2:00}", minutes, seconds, hundredths);
}
(EDIT) Recently realized I can simply use System.DateTime, as demonstrated in this javascript fragment:
var date : System.DateTime;
date = new System.DateTime(seconds * System.TimeSpan.TicksPerSecond);
If you want it as a nicely formatted string, use date.ToString(). This can also take a format parameter to control the formatting details, see MSDN docs for more info.