How can I get the Windows system time with millisecond resolution?

If the above is not possible, then how can I get the operating system start开发者_运维百科 time? I would like to use this value together with timeGetTime() in order to compute a system time with millisecond resolution.

---

Try this article from MSDN Magazine. It's actually quite complicated.

Implement a Continuously Updating, High-Resolution Time Provider for Windows
(archive link)

---

This is an elaboration of the above comments to explain the some of the whys.

First, the GetSystemTime* calls are the only Win32 APIs providing the system's time. This time has a fairly coarse granularity, as most applications do not need the overhead required to maintain a higher resolution. Time is (likely) stored internally as a 64-bit count of milliseconds. Calling timeGetTime gets the low order 32 bits. Calling GetSystemTime, etc requests Windows to return this millisecond time, after converting into days, etc and including the system start time.

There are two time sources in a machine: the CPU's clock and an on-board clock (e.g., real-time clock (RTC), Programmable Interval Timers (PIT), and High Precision Event Timer (HPET)). The first has a resolution of around ~0.5ns (2GHz) and the second is generally programmable down to a period of 1ms (though newer chips (HPET) have higher resolution). Windows uses these periodic ticks to perform certain operations, including updating the system time.

Applications can change this period via timerBeginPeriod; however, this affects the entire system. The OS will check / update regular events at the requested frequency. Under low CPU loads / frequencies, there are idle periods for power savings. At high frequencies, there isn't time to put the processor into low power states. See Timer Resolution for further details. Finally, each tick has some overhead and increasing the frequency consumes more CPU cycles.

For higher resolution time, the system time is not maintained to this accuracy, no more than Big Ben has a second hand. Using QueryPerformanceCounter (QPC) or the CPU's ticks (rdtsc) can provide the resolution between the system time ticks. Such an approach was used in the MSDN magazine article Kevin cited. Though these approaches may have drift (e.g., due to frequency scaling), etc and therefore need to be synced to the system time.

---

In Windows, the base of all time is a function called GetSystemTimeAsFiletime.

• It returns a structure that is capable of holding a time with 100ns resoution.
• It is kept in UTC

The FILETIME structure records the number of 100ns intervals since January 1, 1600; meaning its resolution is limited to 100ns.

This forms our first function:

A 64-bit number of 100ns ticks since January 1, 1600 is somewhat unwieldy. Windows provides a handy helper function, FileTimeToSystemTime that can decode this 64-bit integer into useful parts:

record SYSTEMTIME {
   wYear: Word;
   wMonth: Word;
   wDayOfWeek: Word;
   wDay: Word;
   wHour: Word;
   wMinute: Word;
   wSecond: Word;
   wMilliseconds: Word;
}

Notice that SYSTEMTIME has a built-in resolution limitation of 1ms

Now we have a way to go from FILETIME to SYSTEMTIME:

We could write the function to get the current system time as a SYSTEIMTIME structure:

SYSTEMTIME GetSystemTime()
{
    //Get the current system time utc in it's native 100ns FILETIME structure
    FILETIME ftNow;
    GetSytemTimeAsFileTime(ref ft);

    //Decode the 100ns intervals into a 1ms resolution SYSTEMTIME for us
    SYSTEMTIME stNow;
    FileTimeToSystemTime(ref stNow);

    return stNow;
}

Except Windows already wrote such a function for you: GetSystemTime

Local, rather than UTC

Now what if you don't want the current time in UTC. What if you want it in your local time? Windows provides a function to convert a FILETIME that is in UTC into your local time: FileTimeToLocalFileTime

You could write a function that returns you a FILETIME in local time already:

FILETIME GetLocalTimeAsFileTime()
{
   FILETIME ftNow;
   GetSystemTimeAsFileTime(ref ftNow);

   //convert to local
   FILETIME ftNowLocal
   FileTimeToLocalFileTime(ftNow, ref ftNowLocal);

   return ftNowLocal;
}

And lets say you want to decode the local FILETIME into a SYSTEMTIME. That's no problem, you can use FileTimeToSystemTime again:

Fortunately, Windows already provides you a function that returns you the value:

Precise

There is another consideration. Before Windows 8, the clock had a resolution of around 15ms. In Windows 8 they improved the clock to 100ns (matching the resolution of FILETIME).

• GetSystemTimeAsFileTime (legacy, 15ms resolution)
• GetSystemTimeAsPreciseFileTime (Windows 8, 100ns resolution)

This means we should always prefer the new value:

You asked for the time

You asked for the time; but you have some choices.

The timezone:

• UTC (system native)
• Local timezone

The format:

• FILETIME (system native, 100ns resolution)
• SYTEMTIME (decoded, 1ms resolution)

Summary

• 100ns resolution: FILETIME
  • UTC: GetSytemTimeAsPreciseFileTime (or GetSystemTimeAsFileTime)
  • Local: (roll your own)
• 1ms resolution: SYSTEMTIME
  • UTC: GetSystemTime
  • Local: GetLocalTime

---

GetTickCount will not get it done for you.

Look into QueryPerformanceFrequency / QueryPerformanceCounter. The only gotcha here is CPU scaling though, so do your research.

---

Starting with Windows 8 Microsoft has introduced the new API command GetSystemTimePreciseAsFileTime

Unfortunately you can't use that if you create software which must also run on older operating systems.

My current solution is as follows, but be aware: The determined time is not exact, it is only near to the real time. The result should always be smaller or equal to the real time, but with a fixed error (unless the computer went to standby). The result has a millisecond resolution. For my purpose it is exact enough.

void GetHighResolutionSystemTime(SYSTEMTIME* pst)
{
    static LARGE_INTEGER    uFrequency = { 0 };
    static LARGE_INTEGER    uInitialCount;
    static LARGE_INTEGER    uInitialTime;
    static bool             bNoHighResolution = false;

    if(!bNoHighResolution && uFrequency.QuadPart == 0)
    {
        // Initialize performance counter to system time mapping
        bNoHighResolution = !QueryPerformanceFrequency(&uFrequency);
        if(!bNoHighResolution)
        {
            FILETIME ftOld, ftInitial;

            GetSystemTimeAsFileTime(&ftOld);
            do
            {
                GetSystemTimeAsFileTime(&ftInitial);
                QueryPerformanceCounter(&uInitialCount);
            } while(ftOld.dwHighDateTime == ftInitial.dwHighDateTime && ftOld.dwLowDateTime == ftInitial.dwLowDateTime);
            uInitialTime.LowPart  = ftInitial.dwLowDateTime;
            uInitialTime.HighPart = ftInitial.dwHighDateTime;
        }
    }

    if(bNoHighResolution)
    {
        GetSystemTime(pst);
    }
    else
    {
        LARGE_INTEGER   uNow, uSystemTime;

        {
            FILETIME    ftTemp;
            GetSystemTimeAsFileTime(&ftTemp);
            uSystemTime.LowPart  = ftTemp.dwLowDateTime;
            uSystemTime.HighPart = ftTemp.dwHighDateTime;
        }
        QueryPerformanceCounter(&uNow);
        
        LARGE_INTEGER   uCurrentTime;
        uCurrentTime.QuadPart = uInitialTime.QuadPart + (uNow.QuadPart - uInitialCount.QuadPart) * 10000000 / uFrequency.QuadPart;
        
        if(uCurrentTime.QuadPart < uSystemTime.QuadPart || abs(uSystemTime.QuadPart - uCurrentTime.QuadPart) > 1000000)
        {
            // The performance counter has been frozen (e. g. after standby on laptops)
            // -> Use current system time and determine the high performance time the next time we need it
            uFrequency.QuadPart = 0;
            uCurrentTime = uSystemTime;
        }

        FILETIME ftCurrent;
        ftCurrent.dwLowDateTime  = uCurrentTime.LowPart;
        ftCurrent.dwHighDateTime = uCurrentTime.HighPart;
        FileTimeToSystemTime(&ftCurrent, pst);
    }
}

---

GetSystemTimeAsFileTime gives the best precision of any Win32 function for absolute time. QPF/QPC as Joel Clark suggested will give better relative time.

---

Since we all come here for quick snippets instead of boring explanations, I'll write one:

FILETIME t;
GetSystemTimeAsFileTime(&t); // unusable as is

ULARGE_INTEGER i;
i.LowPart = t.dwLowDateTime;
i.HighPart = t.dwHighDateTime;

int64_t ticks_since_1601 = i.QuadPart; // now usable
int64_t us_since_1601   = (i.QuadPart * 1e-1);
int64_t ms_since_1601   = (i.QuadPart * 1e-4);
int64_t sec_since_1601  = (i.QuadPart * 1e-7);

// unix epoch
int64_t unix_us  = (i.QuadPart * 1e-1) - 11644473600LL * 1000000;
int64_t unix_ms  = (i.QuadPart * 1e-4) - 11644473600LL * 1000;
double  unix_sec = (i.QuadPart * 1e-7) - 11644473600LL;

// i.QuadPart is # of 100ns ticks since 1601-01-01T00:00:00Z
// difference to Unix Epoch is 11644473600 seconds (attention to units!)

No idea how drifting performance-counter-based answers went up, don't do slippage bugs, guys.

---

QueryPerformanceCounter() is built for fine-grained timer resolution.

It is the highest resolution timer that the system has to offer that you can use in your application code to identify performance bottlenecks

Here is a simple implementation for C# devs:

    [DllImport("kernel32.dll")]
    extern static short QueryPerformanceCounter(ref long x);
    [DllImport("kernel32.dll")]
    extern static short QueryPerformanceFrequency(ref long x);
    private long m_endTime;
    private long m_startTime;
    private long m_frequency;

    public Form1()
    {
        InitializeComponent();
    }
    public void Begin()
    {
        QueryPerformanceCounter(ref m_startTime);
    }
    public void End()
    {
        QueryPerformanceCounter(ref m_endTime);
    }

    private void button1_Click(object sender, EventArgs e)
    {
        QueryPerformanceFrequency(ref m_frequency);
        Begin();
        for (long i = 0; i < 1000; i++) ;
        End();
        MessageBox.Show((m_endTime - m_startTime).ToString());
    }

If you are a C/C++ dev, then take a look here: How to use the QueryPerformanceCounter function to time code in Visual C++

---

Well, this one is very old, yet there is another useful function in Windows C library _ftime, which returns a structure with local time as time_t, milliseconds, timezone, and daylight saving time flag.

---

In C11 and above (or C++17 and above) you can use timespec_get() to get time with higher precision portably

#include <stdio.h>
#include <time.h>
 
int main(void)
{
    struct timespec ts;
    timespec_get(&ts, TIME_UTC);
    char buff[100];
    strftime(buff, sizeof buff, "%D %T", gmtime(&ts.tv_sec));
    printf("Current time: %s.%09ld UTC\n", buff, ts.tv_nsec);
}

If you're using C++ then since C++11 you can use std::chrono::high_resolution_clock, std::chrono::system_clock (wall clock), or std::chrono::steady_clock (monotonic clock) in the new <chrono> header. No need to use Windows-specific APIs anymore

auto start1 = std::chrono::high_resolution_clock::now();
auto start2 = std::chrono::system_clock::now();
auto start3 = std::chrono::steady_clock::now();
// do some work
auto end1 = std::chrono::high_resolution_clock::now();
auto end2 = std::chrono::system_clock::now();
auto end3 = std::chrono::steady_clock::now();

std::chrono::duration<long long, std::milli> diff1 = end1 - start1;
std::chrono::duration<double, std::milli>    diff2 = end2 - start2;
auto diff3 = std::chrono::duration_cast<std::chrono::milliseconds>(end3 - start3);

std::cout << diff.count() << ' ' << diff2.count() << ' ' << diff3.count() << '\n';