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先在畫面上拉幾個控制項
TextBox => 輸入「分鍾」，例如: 倒數5分鍾
Button => 開始
Label => 顯示倒數開始
Timer => 跑倒數要用的元件

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        TextBox1.Text = 2
        Timer1.Interval = 1000
        Timer1.Enabled = False
    End Sub

    Dim timer_I As Integer = 0
    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        'Label1.Text = timer_I.ToString()
        Dim dtime As New TimeSpan(0, timer_I, 0)
        Label1.Text = dtime.ToString()
        timer_I = timer_I - 1
        If timer_I <= 0 Then
            Label1.BackColor = Color.Red
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        ' 計算時間的開始(分鍾)
        timer_I = CInt(TextBox1.Text) * 60
        ' 啟動計時器
        Timer1.Enabled = True
        ' 恢復預設值
        Label1.BackColor = Color.FromKnownColor(KnownColor.Control)
    End Sub