def _format_date(pdb_date):
    """Convert dates from DD-Mon-YY to YYYY-MM-DD format (PRIVATE)."""
    date = ""
    year = int(pdb_date[7:])
    if year < 50:
        century = 2000
    else:
        century = 1900
    date = str(century + year) + "-"
    all_months = ["xxx", "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul",
                  "Aug", "Sep", "Oct", "Nov", "Dec"]
    month = str(all_months.index(pdb_date[3:6]))
    if len(month) == 1:
        month = "0" + month
    date = date + month + "-" + pdb_date[:2]
    return date