These docs are for an old release. Info on upgrading to v4
edit doc

formatRange

Formats a date range by intelligently inserting a dash between the two dates.

$.fullCalendar.formatRange( moment1, moment2, formatString [, separator, isRTL ] ) -> String

moment1 and moment2 must be Moment objects.

formatString is a date formatting string written as if it were for a single date, but FullCalendar will intelligently use it to format both dates with a dash in between.

Example:

var moment1 = moment('2013-09-02');
var moment2 = moment('2013-09-09');
$.fullCalendar.formatRange(moment1, moment2, 'MMMM D YYYY');

=> "Sep 2 - 9 2013"