A computer-implemented technique is described herein for rescheduling events within calendars to achieve a target objective with respect to target occurrences of a particular kind in the calendars. For instance, the target occurrences may correspond to instances of free time, and the target objective is to increase the amount of minimum-sized free-time blocks within the calendars, e.g., by favoring the introduction of large free-time blocks of minimum size (e.g., two-hour blocks), and disfavoring the presence of smaller-sized free-time blocks scattered among calendar events. In other words, the technique defragments the calendars with respect to free time expressed in those calendars. At the same time, the technique honors (when possible) participant-related constraints associated with calendar events. Further, the technique penalizes modification of an event as a function of a temporal distance over which the event is moved; this penalization has the effect of reducing disruption to the calendars.