ntp: T5692: add support to configure leap second behavior

docs/configuration/service/ntp.rst

@@ -81,4 +81,33 @@ Configuration
 
 .. cfgcmd:: set service ntp vrf <name>
 
-  Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.
+   Specify name of the :abbr:`VRF (Virtual Routing and Forwarding)` instance.
+
+.. cfgcmd:: set service ntp leap-second [ignore|smear|system|timezone]
+
+   Define how to handle leaf-seonds.
+
+   * `ignore`: No correction is applied to the clock for the leap second. The
+     clock will be corrected later in normal operation when new measurements are
+     made and the estimated offset includes the one second error.
+
+   * `smear`: When smearing a leap second, the leap status is suppressed on the
+     server and the served time is corrected slowly by slewing instead of
+     stepping. The clients do not need any special configuration as they do not
+     know there is any leap second and they follow the server time which
+     eventually brings them back to UTC. Care must be taken to ensure they use
+     only NTP servers which smear the leap second in exactly the same way for
+     synchronisation.
+
+   * `system`: When inserting a leap second, the kernel steps the system clock
+     backwards by one second when the clock gets to 00:00:00 UTC. When deleting
+     a leap second, it steps forward by one second when the clock gets to
+     23:59:59 UTC.
+
+   * `timezone`: This directive specifies a timezone in the system timezone
+     database which chronyd can use to determine when will the next leap second
+     occur and what is the current offset between TAI and UTC. It will
+     periodically check if 23:59:59 and 23:59:60 are valid times in the
+     timezone. This normally works with the right/UTC timezone which is the
+     default
+