How to set a countdown and conduct a comparison of time in visual basic

[sdhillon70]

I currently have an app I am learning with. I design and redesign as I go. I am currently working with Time. The app currently has two timers that display current time local to client pc (As in not just my time) and UTC time.

    Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
 
        If ComboBox1.Text = "Local" Then
            Clock1.ForeColor = Color.DimGray
            Clock1.Value = Date.Now.ToString("HH:mm:ss")
        ElseIf ComboBox1.Text = "Eve" Then
            Clock1.ForeColor = Color.Black
            Clock1.Value = Date.UtcNow.ToString("HH:mm:ss")
        End If
 
        If ComboBox2.Text = "Local" Then
            Clock2.ForeColor = Color.DimGray
            Clock2.Value = Date.Now.ToString("HH:mm:ss")
        ElseIf ComboBox2.Text = "Eve" Then
            Clock2.ForeColor = Color.Black
            Clock2.Value = Date.UtcNow.ToString("HH:mm:ss")
        End If
 
 

Both clocks are displayed in 24h format so HH:mm:ss.

Now I want to allow the user to input either a fixed time or a quantity of time ie( 4Hours or 4 o'clock ) I am thinking it is just a timer with some kind of a reverse tick or a subtraction going on. I am not finding anything on this subject on line. Can someone please point me in the correct direction and perhaps post some relevant links.

I Also need to be able to compare two times and find the difference between them. Can I just do this like a normal math problem or do I need to do something with the time first.

Thanks

~S.Paul