diff --git a/sched/wdog/wd_start.c b/sched/wdog/wd_start.c
index 072d0567500..f41606919cc 100644
--- a/sched/wdog/wd_start.c
+++ b/sched/wdog/wd_start.c
@@ -204,7 +204,20 @@ int wd_start(FAR struct wdog_s *wdog, sclock_t delay,
   up_getpicbase(&wdog->picbase);
   wdog->arg = arg;
 
-  /* Calculate delay+1, forcing the delay into a range that we can handle */
+  /* Calculate delay+1, forcing the delay into a range that we can handle.
+   *
+   * NOTE that one is added to the delay.  This is correct and must not be
+   * changed:  The contract for the use wdog_start is that the wdog will
+   * delay FOR AT LEAST as long as requested, but may delay longer due to
+   * variety of factors.  The wdog logic has no knowledge of the the phase
+   * of the system timer when it is started:  The next timer interrupt may
+   * occur immediately or may be delayed for almost a full cycle. In order
+   * to meet the contract requirement, the requested time is also always
+   * incremented by one so that the delay is always at least as long as
+   * requested.
+   *
+   * This is extensive documentation about this time issue elsewhere.
+   */
 
   if (delay <= 0)
     {