SetSystemTime - UTC or local time?

Question

I have a call to SetSystemTime from my c# application. However if I have the Windows timezone set to a non-zero offset from UTC, it seems sometimes to adjust the system clock as if the time I provided was UTC (i.e. converts to local time) and other times it doesn't, it just sets the time directly to the date parameter.

    [StructLayout(LayoutKind.Sequential)]
    internal struct SystemTime
    {
        public short Year;
        public short Month;
        public short DayOfWeek;
        public short Day;
        public short Hour;
        public short Minute;
        public short Second;
        public short Milliseconds;
    }

    [DllImport("kernel32.dll", SetLastError = true)]
    internal static extern bool SetSystemTime(ref SystemTime st);

    public static bool AdjustSystemClock(DateTime date)
    {
        SystemTime systemTime = new SystemTime();
        systemTime.Year = (short)date.Year;
        systemTime.Month = (short)date.Month;
        systemTime.Day = (short)date.Day;
        systemTime.Hour = (short)date.Hour;
        systemTime.Minute = (short)date.Minute;
        systemTime.Second = (short)date.Second;
        return SetSystemTime(ref systemTime);
    }

The difference seems to be: When I set the time zone using Windows, then start the application, when I call SetSystemTime() it adjusts the time provided as if it was UTC.

But when I set the time zone using the SetDynamicTimeZoneInformation() function, restart the application and then call SetSystemTime() then it sets the time directly to the time I provide, regardless of the time zone.

Is this the expected behaviour? How can I get consistency between the two methods for setting the time zone?

Answer 1

I believe I found the problem.

It turns out the person who wrote the bit of code to SetDynamicTimeZoneInformation() neglected to set the Bias property.

Therefore the timezone information being set had a UTC offset of zero, so no adjustment would take place.