Например https://github.com/freedesktop/fontconfig/blob/master/src/fc...

FcBool
FcInitBringUptoDate (void)
{
    FcConfig    *config = FcConfigReference (NULL);
    FcBool    ret = FcTrue;
    time_t    now;
    if (!config)
    return FcFalse;
    /*
     * rescanInterval == 0 disables automatic up to date
     */
    if (config->rescanInterval == 0)
    goto bail;
    /*
     * Check no more often than rescanInterval seconds
     */
    now = time (0);
    if (config->rescanTime + config->rescanInterval - now > 0)
    goto bail;
    /*
     * If up to date, don't reload configuration
     */
    if (FcConfigUptoDate (0))
    goto bail;
    ret = FcInitReinitialize ();
bail:
    FcConfigDestroy (config);
    return ret;
}

https://github.com/freedesktop/fontconfig/blob/master/fc-cac...

  /* 
     * Now we need to sleep a second  (or two, to be extra sure), to make
     * sure that timestamps for changes after this run of fc-cache are later
     * then any timestamps we wrote.  We don't use gettimeofday() because
     * sleep(3) can't be interrupted by a signal here -- this isn't in the
     * library, and there aren't any signals flying around here.
     */
    /* the resolution of mtime on FAT is 2 seconds */

Надеюсь сам догонишь, для чего кэшу время?