Date parsing: consider referring to RFC 3339 rather than ISO 8601

[JKingweb]

The Value Class Pattern date parsing rules make mention of ISO 8601, but this has some problems:

• The text of ISO 8601 is not readily available
• Whitespace and lowercase "t" are not allowed as separators for date and time in ISO 8601, but widely used in microformats
• Technically ISO 8601 negative timezone offsets should use the mathematical minus sign character (U+2212) when possible, complicating parsing
• Many of the formats in ISO 8601 are mentioned as acceptable in VCP parsing, but not all, leading to ambiguity
• Some of the formats in ISO 8601 (those without separators) can be inditinguishable from numbers, which might be problematic
• The -00:00 time zone offset is undefined
• VCP parsing also accepts formats not in ISO 8601 (e.g. 12-hour clock)

RFC 3339, on the other hand:

• Can actually be consulted freely
• Has clearly defined semantics for all elements
• Allows space and "t" as separators
• Any desirable formats in ISO 8601 and not in RFC 3339 can be explicitly listed as acceptable along with the 12-hour clock