A GedcomX Recurring Date.
function Recurring(str) {
  
  var parts = str.split('/');

  if(str.charAt(0) != 'R' || parts.length != 3) {
    throw new Error('Invalid Recurring Date');
  }

  // We must have start and end. error if both aren't set
  if(!parts[1] || !parts[2]) {
    throw new Error('Recurring must have a start and end');
  }

  var countNum = parts[0].substr(1);

  // Validate count is a number if set
  if(countNum) {
    if(!(/^[0-9]+$/.test(countNum))) {
      throw new Error('Invalid recurrence count: not a number')
    }
    this.count = parseInt(countNum, 10);
    if(this.count < 0) throw new Error('Invalid recurrence count');
  }

  Range.call(this, parts[1]+'/'+parts[2]);

  // If we have a count, replace end with the actual end date or undefined.
  delete this.end;
  if(this.count) {
    this.end = this.getNth(this.count);
  }
}