You could use a TimeSpan to handle calculations on your DateTime values:
Dim myDate1 As DateTime = new DateTime(2013, 11, 18, 4, 29, 38);
Dim myDate2 As DateTime = new DateTime(2013, 11, 18, 4, 29, 49);
TimeSpan ts = myDate2 - myDate1;
long elapsedInMilliseconds = ts.Elapsed;
float valueInSeconds = (float)elapsedInMilliseconds / 1000f;
int value = Math.Floor(valueInSeconds);