求vb倒数时间代码，急急急！

在文本框中输入"01:01:12",按下开始按钮文本框的时间就开始倒数，该怎么完成？急求代码！

Dim a, b, c As Integer
Private Sub Command1_Click()
Timer1.Interval = 1000
End Sub
Private Sub Form_Load()
Text1.Text = "01:01:12"
a = Mid(Text1.Text, 1, 2)
b = Mid(Text1.Text, 4, 2)
c = Mid(Text1.Text, 7, 2)
End Sub
Private Sub Timer1_Timer()
Text1.Text = a & ":" & b & ":" & c
If c >= 0 Then c = c - 1
If c < 0 Then c = 59: b = b - 1
If b < 0 Then a = a - 1: b = 59
If a < 0 Then a = 23: b = 59: c = 59
End Sub

这只是一个例子，自己看一下