The first option is generally allowed by protocols where spaces are appropriate. Using the other alternatives would get one into the business of defining standa

The first option is generally allowed by protocols where spaces are appropriate. Using the other alternatives would get one into the business of defining standards, which no sane person with an appreciation for the subtlety of that task would do unless they had no choice. Having multiple separators or mashing the numbers together would undermine the distinctiveness of ISO8601, and the distinctiveness allows someone to know the precise semantics of a date even when it is taken out of context.