date_is_dst = function (date) {
              var is_southern = date.getMonth() > 7,
                  base_offset = is_southern ? get_june_offset(date.getFullYear()) : 
                                              get_january_offset(date.getFullYear()),
                  date_offset = get_date_offset(date),
                  is_west = base_offset < 0,
                  dst_offset = base_offset - date_offset;
                  
              if (!is_west && !is_southern) {
                  return dst_offset < 0;
              }

              return dst_offset !== 0;
          }
Private method.
Checks whether a given date is in daylight saving time.
If the date supplied is after august, we assume that we're checking
for southern hemisphere DST.
@param {Date} date
@returns {Boolean}