Rebase to 2015a
      - New leap second 2015-06-30 23:59:60 UTC as per IERS Bulletin C 49.
      - The Mexican state of Quintana Roo, represented by America/Cancun,
        will shift from Central Time with DST to Eastern Time without DST
        on 2015-02-01 at 02:00.
      - Chile will not change clocks in April or thereafter; its new standard time
        will be its old daylight saving time.  This affects America/Santiago,
        Pacific/Easter, and Antarctica/Palmer.