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

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.

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';