doc: document leapsecmode directive

chrony.texi.in

@@ -1158,6 +1158,7 @@ the configuration file is ignored.
 * include directive::           Include a configuration file
 * initstepslew directive::      Trim the system clock on boot-up
 * keyfile directive::           Specify location of file containing keys
+* leapsecmode directive::       Select leap second handling mode
 * leapsectz directive::         Read leap second data from tz database
 * local directive::             Allow unsynchronised machine to act as server
 * lock_all directive::          Require that chronyd be locked into RAM
@@ -1808,6 +1809,44 @@ The ID for the chronyc authentication key is specified with the commandkey
 command (see earlier). The command key can be generated automatically on
 start with the @code{generatecommandkey} directive.
 @c }}}
+@c {{{ leapsecmode
+@node leapsecmode directive
+@subsection leapsecmode
+This directive selects how @code{chronyd} handles leap seconds.  The Unix time
+doesn't include leap seconds.  When a leap second is applied to UTC, the system
+clock is off by one second and it needs to be corrected.
+
+There are four options:
+
+@table @code
+@item system
+The kernel steps the system clock backwards by one second at 0:00:00 UTC
+(before correction) when leap second is inserted or steps forward by one second
+at 23:59:59 UTC when leap second is deleted.  This is the default mode when the
+system driver supports leap seconds (currently Linux only).
+@item step
+This is similar to the system mode, except the clock is stepped by
+@code{chronyd} instead of the kernel.  This is the default mode when the system
+driver doesn't support leap seconds.
+@item slew
+The clock is corrected by slew starting at 0:00:00 UTC when leap second is
+inserted or 23:59:59 UTC when leap second is deleted.  This may be preferred
+over the system or step mode when applications running on the system are
+sensitive to jumps in the system time and it's acceptable that the clock will
+be off for a longer time.  On Linux with the default @code{maxslewrate} the
+correction takes 12 seconds.  Note that there will still be a jump in the time
+that @code{chronyd} serves to NTP clients.
+@item 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.
+@end table
+
+An example of the command is
+@example
+leapsecmode slew
+@end example
+@c }}}
 @c {{{ leapsectz
 @node leapsectz directive
 @subsection leapsectz