set clock_interval [<seconds>]
This setting controls how often the client's status bar clock gets updated. The default value is 60 (at the top of every minute). The interval uses a snap timer.
Previously, this value was not configurable and the client would just update the clock (and $T inline expando, /ON TIMER, and /ON IDLE) at the top of every minute. For some people, this was not nearly often enough. Now you can control exactly how often the clock is updated with this /SET.
To get the clock to display seconds and update every second, you might do the following:
/SET CLOCK_INTERVAL 1 /SET CLOCK_FORMAT %H:%M:%S