code: function icaltime_subtract(aDate) {
        var unixTime = this.toUnixTime() + this.utcOffset();
        var other = aDate.toUnixTime() + aDate.utcOffset();
        return ICAL.Duration.fromSeconds(unixTime - other);
      }
docs: Subtract the date details (_excluding_ timezone).  Useful for finding
  the relative difference between two time objects excluding their
  timezone differences.
  
  @param {ICAL.Time} aDate     The date to substract
  @return {ICAL.Duration}      The difference as a duration