// convert between unix epoch and clock_monotonic (ms since boot)
static int64_t g_realtime_to_monotonic_offset_ms = 0;

// In event_loop() at startup:
static void init_realtime_monotonic_offset(void) {
  struct timespec monotonic_ts;
  struct timespec realtime_ts;
  clock_gettime(CLOCK_MONOTONIC, &monotonic_ts);
  clock_gettime(CLOCK_REALTIME, &realtime_ts);

  int64_t realtime_ms = realtime_ts.tv_sec * 1000LL + realtime_ts.tv_nsec / 1000000;
  int64_t monotonic_ms = monotonic_ts.tv_sec * 1000LL + monotonic_ts.tv_nsec / 1000000;
  g_realtime_to_monotonic_offset_ms = realtime_ms - monotonic_ms;
}

static int64_t epoch_to_monotonic_ms(int64_t epoch){
  int64_t const epochms = epoch * 1000;
  int64_t const monotonic_ms = epochms - g_realtime_to_monotonic_offset_ms;
  assert(monotonic_ms >= 0);
  return monotonic_ms;
}