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).
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