lookup_key = function () {
              var january_offset = get_january_offset(),
                  june_offset = get_june_offset(),
                  diff = january_offset - june_offset;

              if (diff < 0) {
                  return january_offset + ",1";
              } else if (diff > 0) {
                  return june_offset + ",1," + HEMISPHERE_SOUTH;
              }

              return january_offset + ",0";
          }
This function does some basic calculations to create information about
the user's timezone. It uses REFERENCE_YEAR as a solid year for which
the script has been tested rather than depend on the year set by the
client device.

Returns a key that can be used to do lookups in jstz.olson.timezones.
eg: "720,1,2". 

@returns {String}