libc: consistency of tv_sec when tv_nsec wraps

The libc's internal calculation of the tv_sec and tv_nsec fields must be
based on the same ms value. Otherwise, tv_sec is not always immediately
incremented whenever tv_nsec wraps. For applications, this inconsistency
can result in observed jumps in time.

Fixes #5098

repos/libports/src/lib/libc/internal/rtc.h

@@ -116,7 +116,7 @@ struct Libc::Rtc : Vfs::Watch_response_handler
 			msecs_since_rtc_update.value / 1000;
 
 		result.tv_sec  = _rtc_value + seconds_since_rtc_update;
-		result.tv_nsec = (current_msecs.value % 1000) * (1000*1000);
+		result.tv_nsec = (msecs_since_rtc_update.value % 1000) * (1000*1000);
 
 		return result;
 	}