diff --git a/lib/libc/sys/kqueue.2 b/lib/libc/sys/kqueue.2
--- a/lib/libc/sys/kqueue.2
+++ b/lib/libc/sys/kqueue.2
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 23, 2021
+.Dd September 30, 2021
 .Dt KQUEUE 2
 .Os
 .Sh NAME
@@ -599,6 +599,12 @@
 .Va fflags
 contains the events which triggered the filter.
 .Pp
+Periodic timers with a specified timeout of 0 will be silently adjusted to
+timeout after 1 of the time units specified by the requested precision in
+.Va fflags .
+If an absolute time is specified that has already passed, then it is treated as
+if the current time were specified and the event will fire as soon as possible.
+.Pp
 If an existing timer is re-added, the existing timer will be
 effectively canceled (throwing away any undelivered record of previous
 timer expiration) and re-started using the new parameters contained in