Firstly, name your controls with descriptive names, even if it is a test application, it takes a little more time but will always save you time in the long run.
Secondly, learn how to use a debugger, you will learn much more from debugging than pretty much anything else, the following links will help;
Debugger Basics | Microsoft Docs[
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Navigating through Code with the Debugger[
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In regards to your question, the following will help;
Dim datStart as New Date
Dim datEnd as New Date
datStart = DateTime.ParseExact(ComboBox1.Text, "HH:mm tt", Nothing)
datEnd = DateTIme.ParseExact(ComboBox2.Text, "HH:mm tt", Nothing)
Dim tsDuration as New TimeSpan()
tsDuration = datEnd - datStart
Hope this helps
Kind Regards