milliseconds.h

#ifndef INCLUDE_MILLISECONDS
#define INCLUDE_MILLISECONDS
#include <time.h>
#ifdef __WIN32
    #include <windows.h>
    #define M_OS_WINDOWS
#else
    #include <sys/time.h>
    #define M_OS_SANE
#endif
uint64_t milliseconds(void) {
    #ifdef M_OS_WINDOWS
        SYSTEMTIME time;
        GetSystemTime(&time);
        return (time.wSecond * 1000) + time.wMilliseconds;
    #endif
    #ifdef M_OS_SANE
        struct timeval te;
        gettimeofday(&te, NULL);
        return te.tv_sec * 1000 + te.tv_usec / 1000;
    #endif
}
#endif

test.c

#include "milliseconds.h"

int main() {
    printf("%llu", milliseconds());
}