Don't use DateDiff ...

Code:
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim date1 As Date = New Date(2010, 12, 31, 23, 59, 59)
        Dim date2 As Date = Date.Now

        Dim hoursUntil As TimeSpan = date1 - date2
        MessageBox.Show(String.Format("{0} hours until {1} {2}.", hoursUntil.TotalHours, date1.ToShortTimeString, date1.ToShortDateString))
        'IF you want whole hours:
        MessageBox.Show(String.Format("{0} hours, {1} minutes until {2} {3}.", (hoursUntil.Days * 24) + hoursUntil.Hours, hoursUntil.Minutes, date1.ToShortTimeString, date1.ToShortDateString))

    End Sub
-tg