Adjusting a time with a variable

I have spent the last few hours reading the forums here and googling / searching the help files for an answer to this. I am rather new to VB, i just stated learning it on my own a couple weeks ago and this is my first program. So any links or books you would suggest would be helpful also.

I am trying to write a build time estimator. I have the resources set via user input text boxes(tbwoodrequiredresources.text & tbwoodcompletion.text), with the output currently displayed in another text box(tbwoodcompletion.text).

What i want is to be able to make "6" set by the tbwoodcompletion.text.

Code:

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If IsNumeric(tbWoodRequiredResources.Text) Then
            If tbWoodRequiredResources.Text > tbWoodCurrent.Text Then
                'This line calculates the estimated time to completion and returns a decimal
                'Value in the form of hour.min
                tbWoodCompletion.Text = (tbWoodRequiredResources.Text - tbWoodCurrent.Text) / _
                tbWoodPPH.Text
                'This line displays the required resources to complete
                tbWoodDiff.Text = tbWoodRequiredResources.Text - tbWoodCurrent.Text
                'Can not figure this section out, so it just displays a hard coded time to
                'completion.
                lbltest.Text = DateAdd("h", 6, Now)
            Else
                'error handling section
                tbWoodCompletion.Text = "0"
                tbWoodDiff.Text = tbWoodRequiredResources.Text - tbWoodCurrent.Text
            End If
        Else
            'Error message if nothing was entered into the text box
            MessageBox.Show("You did not enter a number for Wood Building")
        End If

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As it stands right now the tbwoodcompletion.text outputs a decimal number that i just use a standard 100:60 conversion chart to manually figure out what time it will finish, but i would prefer to automate it when the click event happens it adds the estimated build time to the current system time.

Thanks