ISO 8601:2004 was in need of revision and improvement.

Representations in the standard needed more formal definitions. The document contained numerous instances that were defined by lax adherence, with conflated concepts of expression and representation. For example, in the representation format YYYY-MM-DDTHH:MM:SS, the usage of T is different from Y. T is to be directly expressed, but Y, M etc. are used as placeholders (variables). While this is typically understood by users, it presents a challenge for encoding these representation formats in computer use.

Some rules used in ISO 8601:2004 led to unnecessary ambiguity. For example, the “nominal duration of month” was set to 30 days, but real-life usage differs.

The document also did not provide provision for representing individual date-time components. For example, a “time shift” cannot be expressed on its own (e.g. +0800, UTC+8).

Last but not least, users have been demanding that ISO 8601 “does more” — there are many instances where expressing further meaning is necessary. For example, the need for expressing years beyond 4-digits, negative years, specifying moving days, recurring events are all necessary.