I'm not sure how to interpret the "may be reused" part, especially given that the procedure here ends with !, typically used for effectful procedures. Could it have been that the author wanted to say "The time structure may *NOT* be reused", but made a typo? Same thing with a few other procedures.
I believe that that means that the object passed as an argument may be mutated. See, for example, the sample implementation of the first procedure:
(define (time-monotonic->time-tai! time-in)
(if (not (eq? (time-type time-in) time-monotonic))
(tm:time-error 'time-monotonic->time-tai! 'incompatible-time-types time-in))
(set-time-type! time-in time-tai)