It depends on exactly what you're trying to do - is correct that you want
Dtime = Etime - Stime. You had Etime and Stime reversed. However, now you
might like to know what to do with it. It depends on what you want to do -
if you just want to know the elapsed seconds, and you know it's less than
one minute, you might just use second(Dtime). If there is a possibility of
minutes, you might use minute(Dtime) * 60 + second(Dtime). It's a little
complicated by the fact that the result is a serial time, which is a
fractional amount of a day that has elapsed since midnight.