Change system date programmatically

Question

How can I change the local system's date & time programmatically with C#?

Answer 1

Here is where I found the answer.; I have reposted it here to improve clarity.

Define this structure:

[StructLayout(LayoutKind.Sequential)]
public struct SYSTEMTIME
{
    public short wYear;
    public short wMonth;
    public short wDayOfWeek;
    public short wDay;
    public short wHour;
    public short wMinute;
    public short wSecond;
    public short wMilliseconds;
}

Add the following extern method to your class:

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

Then call the method with an instance of your struct like this:

SYSTEMTIME st = new SYSTEMTIME();
st.wYear = 2009; // must be short
st.wMonth = 1;
st.wDay = 1;
st.wHour = 0;
st.wMinute = 0;
st.wSecond = 0;

SetSystemTime(ref st); // invoke this method.

Answer 2

A lot of great viewpoints and approaches are already here, but here are some specifications that are currently left out and that I feel might trip up and confuse some people.

1. On Windows Vista, 7, 8 OS this will require a UAC Prompt in order to obtain the necessary administrative rights to successfully execute the SetSystemTime function. The reason is that calling process needs the SE_SYSTEMTIME_NAME privilege.
2. The SetSystemTime function is expecting a SYSTEMTIME struct in coordinated universal time (UTC). It will not work as desired otherwise.

Depending on where/ how you are getting your DateTime values, it might be best to play it safe and use ToUniversalTime() before setting the corresponding values in the SYSTEMTIME struct.

Code example:

DateTime tempDateTime = GetDateTimeFromSomeService();
DateTime dateTime = tempDateTime.ToUniversalTime();

SYSTEMTIME st = new SYSTEMTIME();
// All of these must be short
st.wYear = (short)dateTime.Year;
st.wMonth = (short)dateTime.Month;
st.wDay = (short)dateTime.Day;
st.wHour = (short)dateTime.Hour;
st.wMinute = (short)dateTime.Minute;
st.wSecond = (short)dateTime.Second;

// invoke the SetSystemTime method now
SetSystemTime(ref st);