Add a DateTime to your form, then add this code:
Private Sub DateTime1_TextChanged(sender As Object, e As EventArgs) Handles DateTime1.TextChanged
Dim CurrentDateTime As Date
If Date.TryParse(DateTime1.Text, CurrentDateTime) Then
Dim DifferenceOfTime As TimeSpan = CurrentDateTime.Subtract("12:08")
If DifferenceOfTime.Minutes = 0 Then
'* Our time is on the same minute as target, so set the bit
ModbusTCPCom1.Write("05121",1)
End If
End If
End Sub