Convert string to SYSTEMTIME using C

Converting a Date String to SYSTEMTIME (C and C#)

I just want to share a simple and straightforward solution with you. I’m not sure how helpful it will be for everyone, but for those who need it—it’s here 🙂

This example shows how to convert a date string into a SYSTEMTIME structure in C, and then how to work with the same structure from C#.

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The SYSTEMTIME Structure

SYSTEMTIME is a typedef struct defined as follows:

typedef struct _SYSTEMTIME {
    WORD wYear;
    WORD wMonth;
    WORD wDayOfWeek;
    WORD wDay;
    WORD wHour;
    WORD wMinute;
    WORD wSecond;
    WORD wMilliseconds;
} SYSTEMTIME;

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Converting a Date String to SYSTEMTIME in C

The following function takes a date string and converts it into a SYSTEMTIME structure.

Note: The expected date format is: yyyy-MM-dd hh:mm

SYSTEMTIME convertStringToSystemTime(char *dateTimeString)
{
    SYSTEMTIME systime;

    memset(&systime, 0, sizeof(systime));

    sscanf_s(
        dateTimeString,
        "%d-%d-%d%d:%d:%d",
        &systime.wYear,
        &systime.wMonth,
        &systime.wDay,
        &systime.wHour,
        &systime.wMinute
    );

    return systime;
}

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Doing the Same from C#

A good question is: how do we do this from C#?

Below is an example that demonstrates how to call the Windows API function SetSystemTime from C# using P/Invoke.

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Importing SetSystemTime

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

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Setting the System Time from C#

private void SetSysTime(
    short stYear,
    short stMonth,
    short stDay,
    short stHour,
    short min)
{
    var sysTime = new SYSTEMTIME
    {
        wYear = stYear,
        wMonth = stMonth,
        wDay = stDay,
        wHour = stHour,
        wMinute = min
    };

    SetSystemTime(ref sysTime);
}

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Defining the SYSTEMTIME Struct in C#

We define a struct that mirrors the native SYSTEMTIME layout. The layout is marked as sequential to ensure compatibility with the unmanaged structure.

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

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Calling the Function

Here’s how you can call the function from C#:

SetSysTime(2012, 2, 11, 23, 59);

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By externalizing SetSystemTime and defining a matching structure in C#, you can seamlessly work with system time across native and managed code.