code: function(dt, inc) {
          var args = {
                  dt: dt,
                  inc: inc
              },
              result = this._cacheGet('before', args);
          if (result === false) {
              result = this._iter(new IterResult('before', args));
              this._cacheAdd('before', result, args);
          }
          return result;
      }
docs: Returns the last recurrence before the given datetime instance.
  The inc keyword defines what happens if dt is an occurrence.
  With inc == True, if dt itself is an occurrence, it will be returned.
  @return Date or null