To reproduce :

1) Add Microsoft.VisualBasic assembly to your project reference

2) Change PC timezone to : (GMT+10:00) Canberra, Melbourne, Sydney . Ensure PC is set to automatically adjust clock for daylight savings time. (For this timezone, daylight savings time ends at 3am on 4 Apr 2010.)

3) add following code :

    public void SetNewDateTime(DateTime dt)
    {
        Microsoft.VisualBasic.DateAndTime.Today = dt; // ignores time component
        Microsoft.VisualBasic.DateAndTime.TimeOfDay = dt;  // ignores date component
    }

    private void button1_Click(object sender, EventArgs e)
    {
        DateTime dt = new DateTime(2010, 4, 5, 5, 0, 0);  // XX
        SetNewDateTime(dt);  // XX

        System.Threading.Thread.Sleep(500);

        DateTime dt2 = new DateTime(2010, 4, 4, 1, 0, 0);
        SetNewDateTime(dt2);
    }

4) When button 1 is clicked, the PC clock eventually shows 2am, whereas 1 am was expected. (If code marked at "XX" is removed, the clock sometimes shows the correct time of 1 am).

Any idea what is happening ? (Or is there a more reliable way of setting the PC clock from C# code ?)

TIA.