Gets or sets the creation date: xmp:CreateDate, else CreationDate.
public DateTimeOffset? CreationDate { get; set; }
ISO 32000-2 §14.3.3, Table 349, §7.9.4, §14.3.4. An XMP date without a time zone is its local time with a zero offset (XmpDate). A set date is written to the second, in its offset.